I work in fast food, and in my last job worked for a Burger King franchisee - we hired shoppers to order food at our restaurants four times per month (some months four different people, some months two that took turns, the same four-six people most of the time.) We rotated them through each daypart, two weekdays and two weekends. They had a list of things to look for that they had to fill out immediately after the visit and had to time their total service time. We reimbursed them for the food, paid them mileage and paid them $50 per visit. We had eleven restaurants so it was a nice bit of extra income for some of them. We usually found people who went to church with the owners.
I know fast food isn't where you would like to mystery shop, just wanted to give you a real example of it working.
I would say look around at small retail places in your area that might be independently owned and approach them if you are interested. Most of the bigger chains have a program like ours in place as well, it might be listed on their websites?