I don't see much of a point in spending money for special bags. I threw a couple of bags I had around my place in my BOB for this purpose though. One is a small olive drab "man purse" type bag. I actually have no idea where I got it, but it matches my ALICE pack perfectly.
The second is one of those draw string backpacks popular among athletes.
The second is one of those draw string backpacks popular among athletes.