Seems I've read in a lot of places to get a trash compactor bag to insert into your backpack. I've just assumed this is for waterproofing, is that right?

I've looked at a number of different grocers and can't seem to find them. Anyone have suggestions for where to look for them?

I am trying to keep my pack significantly below 30 #, and I'm doing good at that, and I have a smallish pack, so I'd rather get one TC bag than a bunch of smaller stuff sacks or so. Running out of room in my 50L (or is it 45...it seems to be shrinking).

::mattie