Pond liner can be obtained at landscaping outlets. Outdoor fishponds need a large rubber liner. The liner is available by the yard much like material in a fabric store. It's like very thick inner tube rubber. Actually, an inner tube from a large tire like a grader or skidder might be thick enough.
With regard to the seal, the original plastic seal had become thin and brittle, and was leaking badly. I removed the old seal, traced it onto the pond liner, and cut it out. Worked like a charm.
If you need more info, I'll borrow my daughter's digi and post a few shots of the seal.
Rick