Had a couple of families getting ready for there county fair asking about what soap we use. I really like to use non-ultra Joy from the dollar stores (cost $1). It works great and it's cheap but make sure it's the non-ultra. It leaves no residue and keeps the hair healthy.
The other thing I like to use is the Foamer. Throw in the soap and hook it up to the hose and foam away. I always apply the soap with the Foamer when the hair is dry. It soaps up nice as you scrub and rinse. The Foamer really does save on soap to.