Where can I get a bag to brine my turkey in?

Whole Foods.

Any plain old grocery store sells Reynolds Oven Bags, which can be used for brining. You will want the "turkey size." They're usually sold in the plastic wrap/wax paper, etc. section and you can find them all year round.

I just saw some at my local Whole Foods

You can find them at Williams Sonoma, Bed Bath and Beyond, or at this time of year maybe even a local grocery store!