The day I decided to build a pond I called up DH who works for Firestone
Textiles. Asked him to see if we could order some Firestone EPDM in. He
called back a short time later to inform me that the factory was currently
being re-roofed. We got all the rubber roofing (used) we wanted for.....
you guessed it --- free. We had a friend pick it up on a large truck. I
power washed both sides checking for holes etc. The main chunk we put in
the pond had a 1" slice but was easily patched with the roofing cement and
another piece of liner. We do have fish and snails and tadpoles and frogs
and other assorted pond critters. They seem to like it fine. And as
Michael said the stuff is so strong. One day I was trying to trim off a
piece and took a pointed shovel and stood on it hoping to cut it? Didn't
even dent it. The downside is that it is so thick that it does not fold and
flatten easily so there are places where the liner sticks out. I placed the
folds away from the expected water flow. Maybe with time they will fold
against the sides.

You might try driving around looking for a roofer doing a job on a flat roof
and ask if you can help him out by disposing of some of the used rubber for
him... Worth a try and the price is certainly right.

Heather