I bought it retail at a Canadian hardware chain. Don't know who carries it in the US, but somebody probably does since it's made in USA.

(I also bought their low-temp silver solder paste for field repairs and putting tabs on batteries, which can be destroyed by regular solder. This stuff claims it only needs Bic level heat to activate.)

What kind of plastic is your tank made of? High density poly? Maybe I can do a test for you.

Previous comments about plastic welding are valid also, though I have seen some problems if the weld is exposed to a lot of UV.