I've had some success with bentonite clay in a couple of ponds. Your local farm store should have it in 50-pound bags for about $10 a bag.
The small pond I have now has shale in the bottom of it and bentonite didn't work in it. A couple different people have told me that drywall will seal a pond so I'm trying that now. I got a utility trailer full of drywall from a nearby house that is under construction and covered the bottom of the pond. Now I'm waiting to see if it is going to help.