I can't remember for sure, but I thought I saw them at Walmart. Of course, I wasn't looking carefully because it was probably more of an "oh, there are those awful training pants that didn't work for us".
I have used the Motherease waterproof trainers, and the Kushies. You can get the Kushies in Sears and BRU, iirc, but I don't remember how expensive they were.
Also, are you looking for just absorbant underwear, or with a waterproof cover?
Hope someone else can help you!