Pete - I bought bags of bentonite from Palmers Feed mill on Rt 20 in Lafayette. They get it from the big feed mill in Baldwinsville (I think mercers is the name). It came bagged in 50 or 60 lb bags and was pretty reasonable. Guess it depends how big an area your talking about. If your talking about a BIG pond it would be cheaper to go with screened clay like we used for an arena base. Got a few truck fulls of that from Action Topsoil in Minoa. That would work pretty well as a liner but you have to pump the pond out first to use either method.