Generally, you pay an artist to draw something for you. Alternatively, you could contact an artist with a pic you like and ask to buy rights to use it from him/her.
Recipts and documents - you don't have to have anything, any more than if you buy a Snickers bar you have to have a receipt or a document saying you can eat it. The purpose of such documents are to resolve later disputes - because you have proof in writing. It makes sense to have something in writing.
Prices vary massively, depending on the artist. Your best bet is to look around a few publisher sites and see what they pay for art. Art in the EN World Player's Journal, for example, is $25 for a quarter-page black and white piece, and $80 for a full coouor cover. These are fairly low.