The quickest way to find companies shopping your area is to sign up and check jobslinger.com. The signup is free and you will see the companies that have posted jobs recently in a radius from your zip code that interests you on the SASSIE system, the job board at Volition.com and on the MSPA job board. There may be some other boards they are drawing from, but these are the basics.
The jobs you see will have been taken already more than likely, but it will give you the companies to sign up with first.
Recognize that Jobslinger.com draws primarily from the SASSIE system (I think it is run by the same folks), so you will not see jobs from companies near you that post using ShopMetrics, Prophet4, their own software or ClientSmart unless schedulers have ALSO posted jobs on the Volition or MSPA job boards.
In the long run you need to sign up with all the companies you can find and make sure that you are set up with them to notify you by email. This month I am working with a company that has had nothing in my area for several years, but they now have a client. I haven't been checking their job boards regularly because they have had nothing. I have kept my profile alive by updating it from time to time. By email I found they had a new client in my area. But with about $50 in fees this month from them on my job sheet, they are getting moved into the group of companies to check their job board regularly.
The list of companies on this forum is in the first post of the first thread of the Mystery Shopping Company Discussion section.