Be realistic. A three- by five-foot painting by a student starts at $500; expect to pay $5,000 and up for the work of an established painter. Sculpture costs more; silk screens, less.
Once you’ve picked an artist (try an online registry, like wwar.com), ask for sketches of your project (always return them). Don’t be afraid to ask for changes. When you’re happy, sign a contract spelling out expectations and payment.
Have patience. A piece of artwork can take a few months–or more than a year–to complete. But just think how fabulous it will look once you finally get it.