Flash is correct when she says this forum is the best source of information about mystery shopping. This is so because it is for shoppers by shoppers. It is real and also free.
Check out the 'sticky' threads, read about the companies listed in the Official list of Companies linked at the bottom of every forum page, and peruse the articles in the Mystery Shopper Magazine also linked at the bottom of every forum page. If you get into reading what is here at the forum you will see how to identify and avoid the scams, sign up with legitimate companies, and get great advice about shopping in so many ways. It is like a course or seminar right there for the taking.
Good luck and have fun with it!
Happily shopping Rhode Island and nearby Massachusetts and Connecticut