The sad news is, there’s no such thing. Add to that, most booksellers have limited policies when dealing with or representing self-published books. Their reasons are good: 1) most small publishing houses don’t accept returns, the bookstore could get stuck with books they can't sell; 2) while there are exceptions, most self-published books have an amateur look to them; 3) it can be more hassle to deal with people who don’t know the industry than its worth.
     So if traditional bookstores won't carry them, self-published books are rarely noticed unless accompanied by serious advertising. In traditional publishing, companies spend millions of dollars using their already established media platforms to get their books noticed. Unless a writer is already out there in some large marketing capacity for their core career, their self-published projects have an even harder time reaching the marketplace's radar.
     As a result, whatever you pay that illustrator before publication is most likely all they’ll ever see from your project. You can negotiate royalties, but in self-publishing, royalties should be thought of as an unexpected bonus, not fair compensation for the job.
     “Good exposure” is another term illustrators hear too much of—the joke being that you can die from exposure. Hiring an illustrator is a business arrangement like hiring a painter or a stock broker (who would never work for free or for good exposure). It's a professional relationship like any other and requires a contract.
     In the contract will be listed several key points. The first point will be the amount to be paid and when those payments will be made. (1/2 up front, 1/2 upon acceptance of final art, or 1/3 up front, 1/3 on acceptance of sketches, 1/3 upon acceptance of final art are standard arrangements.)
     Another key point is the matter of copyright—who will own the final artwork. Typically the copyright stays with the illustrator (copyright applied and paid for by the publisher). However, you can purchase the copyright to the artwork from the illustrator to be able to do whatever you like with it—this is called work-for-hire. Expect to pay more for this. Although honestly, you don’t need to hold the copyright. It’s one of those misconceptions of hiring an artist—that you have to own the artwork to use it. The contract should spell out exactly how the usage breaks down, both for you and for the illustrator (use of the artwork for the illustrator's self-promotion should be expected and allowed).
     To be fair to the illustrator, there should be a limit to the number of revisions permitted at each stage of creation. (No more than three revisions per sketch is reasonable.) Few revisions should be made after the completion of the art. (If communication has been good, the client should have a good understanding of what they will receive.)
     This avoids the illustrator being hired as ‘hands’ for a person who can see the images in their head but can’t draw themselves, which is an untenable and unrealistic scenario and can lead to countless revisions sucking up any benefit of compensation by requiring too much time. “I’ll know it when I see it” is never a fair way to work unless you are willing to pay extra for revisions above and beyond the limit. (Again, this should be in the contract.) Ideally, you are hiring an illustrator to bring their unique vision to the project.
     That said, sometimes, things don't work. It's rare, but does tend to happen more often with inexperienced creators. (You could hate the illustration direction or the illustrator could lose enthusiasm/patience for the project.) It's why there should also be a kill fee in the contract. If for whatever reason you or the illustrator needs to leave the project, an amount should be agreed upon as to how much the illustrator will keep for already completed work. The 1/2 or 1/3 downpayment is standard depending on how deep into the project you get. This is a circumstance you hope never will occur and certainly, nobody goes into a project thinking this way, but sometimes it can't be avoided.
     Also, understand that because so few self-published works reach popular success, they typically cannot further an artist’s career. They might make for some nice portfolio pieces or be good practice for a younger illustrator. But the value for the illustrator on self-published projects, if they are willing to take them on, lies in the cash flow, not buying into the concept.
     And finally, it is only polite (and traditionally expected) to give the illustrator at least five to twenty copies of the final product free and clear for them to either use for advertising or to keep for whatever reason.
     The bottom line is, approach hiring your illustrator like you would any professional. Do your industry research, make sure you understand the terms I’ve used above, and walk in with a budget. Never say, “I don’t know, how much would you charge?” Most illustrators have worked with clients from all sorts of backgrounds and recognize the signs (red flags) of those who have not done their homework and may be too difficult to work with to be worth their time (usually because of inexperience or unrealistic expectations). Don't let that be you! If you come prepared, you are much more likely to land the illustrator you really want and achieve a successful product.
     Bottom line, self-publishing is a growing industry. When approached professionally, it can be a wonderful experience for all involved.

Questions You Should Be Able to Answer Before Approaching an Illustrator
1) Name of book:
2) Subject of book: (don't worry about anybody stealing your idea - ideas are like noses, everybody has one, and they can't be copyrighted)
3) Genre of book: board book, picture book, non-fiction, chapter book, mid-grade novel, young adult novel (If you don't know what these are, spend some time in your local library or bookstore. Learn the page count expected in each genre.)
4) Illustrations needed for: entire book, spots, cover
5) Illustration budget: be realistic
6) Illustration terms: advance with royalty, flat fee, or work-for-hire
7) Copyright of artwork will go to ____
8) Time for project completion: ____ months
9) Limit of revisions contractually permitted per illustration per stage: (no more than three at sketch stage and one at color stage is acceptable)
10) Payment arrangement:
11) Use of artwork:
12) Number of copies to illustrator:

Please also read How Do I Get Published and Illustrating Self-Published Books and follow the links at the bottom for more helpful information.

Read Sarah McIntyre's 'Can you illustrate my book?' Some tips for writers approaching illustrators

From Muddy Colors: Art for Exposure - Why an illustrator should NEVER work for free.

Illustrator Will Terry offers great commentary on the topic: 7 Reasons I Can NOT Illustrate Your Children's Book