I've never seen them OTC. I did hear someone mentioning ( I think it was on Autopia) they found some at either Costco or Sams Club. It included the sprayer and the bottle. Of course this was after I had already purchased some online.
Sams Club sells a pack of 6 (I think) 32oz bottle and IIRC three of them have foaming sprayers and the others regular.
I always thought about having a pressurized foaming sprayer for wheel wells and tires. This is one i found recently: Amazon.com: Hudson Foam-A-Matic Sprayer - 2 Gallon, Model# 90192: Home Improvement