i buy mine at walgreens (because i work there). and i've found that the walgreens brand underpads for older adults work just as well as the pet disposable pads. you can get 40 pads for about 7.99.
i also use the washable ones when i'm at home, but on the road when i'm travelling, disposable is what i grab (it pretty much lasts me all year since mocha prefers to go out to do her business).