You should be able to get it from a welding shop that specializes in Marine work. Barring that, if you have a Hot Rod shop near you, ask them where they get their Stainless, and their Anodized Aluminum.
For my purposes, and with a very small amount of money to use, I got ahold of a couple sheets of tin along with a combination of angle iron and metal fence posts. Had to weld a bit. Of course corrosion was a concern but a few layers of tanemic (possibly misspelled) paint and I haven't had to worry a bit. Total cost with man hours was approximately 250 USD. Sounds like you need something more like a baffle than a weir.
Professional plastics has 1” hdpe sheets it’s weld able be easy to cut weir but with the price of s.s. I’d use mild steel and a two part epoxy paint with s.s. hardware