I have no idea where to find a brining bag, but I do have some suggestions for what to do as an alternative.
In the past, I've just made do with putting my bird into a sanitized & cleaned paint bucket (the big white 5-gallon types). You should be able to find one of these at B&Q or another home improvement store.
This year I'm going to try the method listed on Slate since I don't have the facilities to do a proper brine. http://www.slate.com/id/2178010/ They suggest 'curing' it with a salt and sugar mix overnight, similar to what you'd do to make gravadlax.
Hope that helps a little.