Kristin,
There are many good companies, but not all of them are in your area. Additionally, most shoppers work for multiple companies because each shopper can't or won't get all the shops offered by one company.
To find companies in your area, go to Mystery Shopping Providers Association (MSPA) at mysteryshop.org and jobslinger.com to search for shops in your area. Each of these sites has a handy function listing them by distance from your zip code.
When you find some listings that interest you note the company. Then apply to these companies - the more the better. You can find out more information about the companies on these forums.
Don't be discouraged when you discover most of the jobs listed at MSPA and jobslinger have been assigned. Members of the companies get emails of shops and look frequently on the websites for shop listings, so these lists get a little dated.
The point is to sign up for many companies so you too will get the emails and be able to search the company's sites for shops more quickly.
When then happens you will be able to take shops. For more detailed information, search these forums. You will find a list of about 250 mystery shopping companies, advice on how to keep a log of shops, and much more.
Have fun!
PS: Never, ever pay to join a mystery shopping company. Why? The legitimate ones are free.
Happily shopping Rhode Island and nearby Massachusetts and Connecticut