How big of a turkey?
I've used 2 gallon ziploc-style bags and I've used the oven roasting bags as a brining bag.
Both are readily available. The 2 gal ziploc I purchased were at Wal-Mart or Target.
Note: For the oven roasting bags, I recommend using a couple twist-ties.
For a bag that's labeled as a "brining bag", try a place like Beds, Bath and Beyond.