I use the same trick, they are cheap, light, and if they rip with use, just toss them. I especially like to fold up a topo map in a zip lock when canoeing, put it down on my pack to glance down every once in awhile, and it stays dry no matter how hard it's raining.
Another good source of larger w/p containers are 5 gallon plastic buckets. A great way to store your grub, and they double as a nice camp stool.