Do you mean for artwork? If you're willing to spend a bit more, I'd go on DeviantArt and search for the keyword "commission" or "com." This will show you artwork by people who do commissions. Most of the commissions on DeviantArt are personal, and they fill up fast. A book is a commercial commission, so if you contact them, and are willing to pay a bit more, most will work with you.
If it's self-published, and your not making much money off it, I'd (very politely) point that out, as most artists will cut you a deal.
DeviantArt also has work for hire and job services forums.
Please don't do this. Pay them the going rate regardless of your expectations for sales.