Before you get flamed for what you logically assume is a reasonable question, let me help you out.
We can't/won't tell you which companies to sign up with because:
-Listing the specific MSC (Mystery Shopping Company) for a client violates our ICA (Independent Contractor Agreement)
-It also violates forum rules
-In a way, helping you is helping a competitor if you're in our same market. Folks are pretty passionate about this and would encourage you to do your own research and apply, apply, apply.
That said, do some research through the search tool, and you should quickly find companies that do the types of assignments you're looking for.
Also, read, read, read. Take advantage of the New Mystery Shoppers area. Read all articles from the user "Flash".
If you lurk for just a bit, I think you'll enjoy your experience on this forum.