For a pond of that size bentonite shouldn't be too expensive. The idea is to mix it thoroughly with the top few inches of clay lining your pond, either raking it in to loose soil or tilling it in, then add the water. You have to have the right kind of soil though, as bentonite reacts improperly with the wrong types, and if your soil doesn't have enough clay then it won't work either.
There again, for a pond of that size a liner wouldn't be too costly either. I installed a big liner last year, and I hated it. We had to first pick all the rocks from the pond floor bigger than 3/8's of an inch then we had to line the whole surface area of the pond with used carpet (I killed my biodesel vw pickup in hauling it all out to the farm but that is more to do with my mechanical ineptitude than anything to do with ponds) to protect the liner against rocks that might work their way up through the soil, then stretch the liner into place, and then lay carpet around the top edge to protect any exposed liner against the ever intensifying UV. You have to dig an anchor trench all the way around the pond to hold the plastic up, and somehow not kick rocks in between the carpet and the liner as you backfill. If you do use a liner, my advice would be to get everything in place, then fill the pond, then backfill your anchor trench, to allow for stretch when the pond is full.
If bentonite would have worked with these soils, and I could have found some locally I would have tried it instead of plastic any day.
caretaking 14 acres of ridge and gully land at Huelo, Maui. 400-500 ft above sea level