LoneGoose,

I think you can simply call an excavator or two. Sometimes when they have a job they will strike clay, especially here in Upstate. In fact in some cases they may be looking for convenient places to dispose of it elsewhere off site (think fill), but then you are relying on luck for good material to be found for your pond. You will need to be very specific to make sure you don't wind up with clay mixed with asphalt.

But yes, while clay is valuable for ponds, it isn't in high demand enough to be sold regularly. Excavators around here are your best bet, it certainly doesn't hurt to try.

Where about are you in NY?