Sign up at Jobslinger.com (it is free) and put in your zip code. Give a reasonable distance you would be willing to drive and see if any companies pop up. Those are the companies you should sign up with first. The jobs currently on the board are likely to have been taken, but you will know whose job boards to check in the future. Then extend your distance and see what comes up. Market Force (the Blue Portal) and Corporate Research International are likely to have shops in your area as they have clients who seem to have locations everywhere. The shops don't pay well, but at least it is some kind of work and if you watch the Corporate Research jobs during the month, you will note that the fees go up towards the end of the month if no one has taken them.
Depending on how rural your area is, you may find that merchandising works better for you than mystery shopping or doing some combination of shopping and merchandising. My understanding is that merchandising pays mileage in addition to paying sometimes by the hour and sometimes by the job. More information about merchandising is at Narms.com