I just bought some generic forms at the local "Office Depot" store. It would probably look more professional to have custom made ones, but I'm trying to keep overhead costs down.
It's cheaper to use the regular ones from the office supply store, have a personal stamp with your info made, and stamp every receipt,invoice ect that leaves your business. Beats the price of having to order cutom made ones each time. ( can get regular ones fron the dollar store for .79cents.)
As a sales reciept I also just bought a book from Office Depot.
if you have MS Office (word, excel) -- they have lots of templates that are easy to modify on the microsoft site: