I use those free plastic bags from Walmart and grocery stores for garbage bags but they always seem to be floating around and trying to get away. I took a empty paper towel tube and stuffed it full of the bags (I can get about 8 in) and now my bags are under control. If you really want to save space, stuff a tube with the towels still on. Now that I think of it, I'll do the same thing for my dog poop bags.
Cost = free!