Stop by REI and buy some drybags and toss them into your backpack if you want cheap. If you want something a lot more legit, look up the Kriega R30, it's pretty hard to beat but it's pretty expensive.
the R30 isnt waterproof....the liner is waterproof, and its removable.
had i known that..... i might of never really bought it because i never use the liner. i thought it was sewn into it like a waterproof glove might be.
I droped the waterproof liner the r30 uses and picked up a sea to summit pouch. THe R30 liner is a really good one, but everyday use with it, would destroy it, and its too bulky to carry around all the time in a seperate pouch.
