I get my washable pads from here:
Buy Waterproof Pads for Whelping
They have sizes from 17 x 20 to 48 x 60. I buy them in bulk and the brown ones shown are the ones I have. They are GREAT. I have a 36 x 36 crate so I buy the 41.5 square ones and just put them down and sit the crate on top. I also put one at the back door, in case I don't notice that they have to go out. Works great for me. Mine have been washed about a million times and still look like new.
They also sell used pads
Used Waterproof Pads and I have some of those too.