Even though we used a layer of clay there's one sandy spot that is leaking.

We have a member that sells product and also provides free expert advice.
Here is a link to a thread where TJ is one of the posters. You can send him a private message by clicking on his name in the thumbnail, or go to his email in one of his posts in that thread.
Pond Leak ThreadHe might be very busy right now, so be patient on waiting for a direct reply.
Good luck on getting your new pond fixed!