The ones photochick mentioned are the ones I use (BestMark, MarketForce, Corporate Research International), and I also do TrendSource, GapBuster, and Second To None. They all have their pluses and minuses - so totally agree that you have to either get a great shop fee or really like the product - I was doing so many Sonic and McDonalds shops for a while there, I was SO burnt out on the food, and there are a couple of shops I won't do anymore (too many photos that have to be *just so*, even if the condition is impossible to get in a photo, and shops rejected unpaid because of it). I've done just about every kind of shop these companies offer - phone, internet shops, e-mail shops, buy and return, consignment sale, eating, etc.
Other tricks I've learned (we used to do a LOT of shopping - haven't had time to do it much since we moved to Atlanta) - try to group your shops geographically, and ask for distance bonuses if it's a long way away. I used to get a distance bonus for a shop in City A, another bonus from another company for City A, and a third bonus for City B, which was on my way home from A, from a third company. Sometimes it meant I spent all weekend in my car, bouncing from shop to shop, but when the pay came in, I'd have made $200-300 in a month, had lunch on the company twice, bought groceries and reimbursed for them, had a new sweater and some lawn fertilizer, and all of it essentially free to me.
Also - remember that this is a JOB and not just for fun, so take it seriously, or you risk not getting paid. print out your questionnaires, and fill them out in a parking lot nearby after your shop. Get a folder and clip receipts/notes to the questionnaires. Take twice as many pictures as they as for. Get names, even if you're not sure you'll need them, if you can do it without getting 'caught' as a shopper. Have a convincing back story for your scenario.
I loved shopping professionally. I wish I had time for it again. We had a friend that lost her job, and using some savings for 'seed money' for her first month's shops, and saving part of each month's shop money for seed money for the next month, she was able to make enough to cover her salary. She was working 8+ hours a day, 6+ days a week, though.
Good luck if you get started - and feel free to PM me if you have any questions.