i bought a box of them a year ago, i have even cut them in half and used them that way. water proof is a good thing!
i got mine at either staples or office maxx.
i used to get bags at "bags unlimited" an online shop.
what they look like..
http://www.google.com/search?q=pictu...=com.microsoft:en-US&ie=utf8&oe=utf8