I don't know. I have never had to buy clay.
Are you a pond builder? Have you done any other improvements on your new property and excavated some better "core trench" material?
If you own (or commonly rent) the heavy equipment, you could move some good material on your own property a short distance pretty cheaply.
OTOH, a dump truck load of clay from a nearby new housing development where they are digging out basements, could probably be diverted to your property at an affordable price.
[I am a geologist, NOT a pond builder. However, I hate the rocks in your leaking dam. No amount of compaction can seal imbricated rocks.]
