Don't bother getting a preformed pond. IMO, they're overpriced. If you're looking for a great deal, try buying a cheap pool. Heres a 2500g pool I found at wal-mart.com
http://www.walmart.com/catalog/product.do?product_id=5693282
Im not exactly sure if its fish safe, but I'm pretty sure it is.
Just don't use the filter that came with it.