At the bottom of each page of the forum is this forum's "Official List of Mystery Shopping Companies". There are links there to conversations about the companies as well as links to register. Since many of these companies may not immediately have work in your area I would encourage you to register with the free version of Jobslinger.com which is a job board that draws from several sources and can be sorted for shops near your zip code. The shops you see may have been taken already, but it will let you know what companies have clients in your area.
Realize that companies pay for work anywhere from 2 weeks to 3 months from the time you perform your shop and many shops require a small purchase which they will reimburse. I mention this because mystery shopping will probably require the outlay of more cash in your first months as a shopper than you are receiving back in payments and reimbursements. Once you have been doing it for a few months and the payments begin coming in, it will get to be a more feasible activity.