There is a conflict intrinsic to your request. There are good mystery shopping companies and there are companies that always seem to have jobs available.
Companies such as Market Force, Corporate Research International and Bestmark almost always have jobs available in almost every market in the US. This is because they have as clients companies that exist in almost every community. The jobs stay on their boards because they (or their clients) choose to pay fees so abysmally low that they are not feasible to perform as a business proposition. Yet eventually there are shoppers in such dire straits that they will do them anyway. But it is not a businesslike decision to do all that is involved in a job--qualifying for it, reading the instructions, going to do the shop, making required purchases for the receipts, reporting the shop and all the paperwork and bookkeeping involved--that uses even a half hour of your time for $4. You may think that $4 for a half hour doesn't seem bad when it only takes 15 minutes on site, but you also have your expenses of going to the location, being out the money you advance for purchases for the weeks or months it takes to get paid, spending your time and equipment at home both for preparation before the shop and the reporting, follow up and storage of notes/data.
There are many good companies out there that may or may not have shops in your area and who post jobs that enough shoppers feel are fair payment for them that they will get taken very quickly. If you are planning on doing shopping as a business you will need to identify those companies in your area that have decent jobs and over time figure out when they are likely to post jobs in your area so you can take them before someone else does.
The way to start is to look on Jobslinger.com and see what companies have posted jobs near you, sign up with those companies and start haunting their job boards. If you decide you like the work, then go on to the lists such as are posted here, on the Volition.com website and the MSPA site and sign up with a lot of companies.