If you go online and search for custom polyethylene tanks you will find lots of suppliers. If you look around you will probably find someone in a nearby city that makes them, or you can use one of the larger firms that are present on the internet
and have them shipped in. These days they don't make welded poly tanks for fuel
(in the US), but water and holding tanks are readily available. Just remember that they will be welded from flat sheets
of plastic so don't expect curves. Curves can be approximated by several flat sections, but at a cost: complexity increases the labor and price
. You should make careful drawings of what you want first, then submit them for quotes to a few suppliers.