I've used Kern, mysteryshops.com, and ShopnChek with success. They are all legit, fairly straightforward, and pretty easy to use. But I agree - I live in a smaller city and there aren't a lot of opportunities that are posted for my area. In a month, I might complete 3 or 4 mystery shops between all of the companies. There seem to be more shoppers than shops around here - they fill up very quickly.
And, although I enjoy it, it isn't as exciting as the spam e-mails make you think. It really is a lot of work to prepare (read the materials, schedule the dates), then during the shop you have to remember all of the questions and procedures as well as the responses you get, and finally you have to go back home, fill out the paperwork, fax or scan the information, etc etc. All in all, most of my local shops pay a shopping fee of $5-$9 plus a $1 reimbursement for a small item you purchase (so that you obtain a receipt to verify the shop). The better shops are restaurants, because you do get a free meal for most of them, but they aren't that common here. I had one shop at a shoe store. When I was browsing the job site, I saw that you got to make a purchase of shoes and I thought Woo-Hoo!!! But then I read further and saw that part of the shop was to return the shoes that same day. Boo.
Good luck!