I'll send you a list of the places I use (that have paid me) if you PM me your email address. You just fill out an application and post the shops you want... pretty easy really
I had good luck with Volition.com. They list the companies that hire shoppers and their message boards give you information on which ones are good and which to avoid.
ditto what Zip said. If you want to do mystery shopping, you have to go to www.volition.com, it has a list of companies and a message board that gives info on which companies to work for. Just realize you have to apply to LOTS of companies.