The more I thought about tubes, the less I liked the idea; figured them being molded in the shape they are might create problems getting smooth, flat edges. I found & ordered some EPDM rubber sheeting, thanks to a suggestion by @ChickenCanoe@kwhites634
Whites if your still looking for an inner tube try a tire shop. They may have some blown out ones you could have. Assuming they still have inner tubes. When we were kids we'd go to them to get our tubes for snow tubing. Fix them up and away we'd go. Free fun.