I hope you realize that this is not a "small job" that many (knowledgeable) artists will do for free. You will be selling this book, correct? This means, indirectly, you're also selling the artwork on the cover. This means the artist has to sign over all rights of the artwork to you, or receive a percentage of the profit per copy sold.
Most artists would accept this, but as a commission. So, what's your budget? Listing this in your "ad" would prove beneficiary to professional artists who have "been there, done that" with book covers. Considering all of the copyright involvement, I wouldn't accept a free piece of artwork from anyone unless it was clear -- in writing -- that they're relinquishing all copyrights to you (artist doesn't get a cut, and you also have proof that they are allowing you to use the work). With the writing... make them sign it!! Don't accept an internet e-mail saying "ok, you can use this." If your book "explodes," they may get a little envious and sue you saying you're using their work without notice/payment. The "contract" you should have will say otherwise.
Regardless, I may be able to make some recommendations if you'd like. Just send me a message or e-mail (email@example.com) with some more details and I'll get back to you ASAP with a list of artists I think would suit your needs. :)
"Adapt or perish, now as ever, is nature's inexorable imperative." (H.G. Wells)
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