PUL is a fabric that has been laminated or treated on one side to make it waterproof, but still soft and stretchy. Most people that make diapers for their skin-kids (human babies) use PUL, that's where I got the idea to use it. This makes the belly bands completely washable so you don't have the added expense of pads or diapers every week if you build in a soaker pad. I make my girls panties with it too.
So far, I have only been able to find it at JoAnn's fabric store and online. Here in KY, it costs about $12 per yard. But JoAnn's mails out coupons for 40% off about once a month so that knocks it down to about $7. And that really isn't bad considering how many belly bands you can get cut from a yard.