Crib pads can be found in the baby section of any store, I got mine from WalMart. They measure about 18" by 24" and are made out of a terry cloth/cotton mix. Basically, they are pads used in a baby's crib to catch "accidents". As such, they are very absorbant and resist smelling, despite the fact that they are made out of terry cloth.
If you are unable to find them, you could probably just make your own. Sew one layer of cotton batting between one layer of cotton fabric (you can use an old thick bedsheet for this, buy one used from a thrift shop, etc.), and one layer of flanette or thin terry cloth material (not a towel, but more like a kitchen dish towel). Quilt the pad you just made so that the batting does not shift when washed, and there you go: a home-made crib pad made to the dimensions you need! The only downfall of the store bought ones is that they are slightly too short for a cage, and so you end up using about 6 per cage change BUT they are very inexpensive ($2-3 dollars per pad) so it's not so bad. I'm looking at making my own once my bank account recovers from my fleece purchase