A really good place would be Tri-State Rotational in Hudson, Indiana. They will not make a custom size but make fresh water tanks and grey/black water tanks for numerous travel trailer builders. The owner also sells his smaller tanks on ebay. The larger tanks cost a lot to ship but he will ship them if you want. I searched for tanks for a long time until I found this place. I recently just purchased six 40 gallon fresh water tanks and two 95 gallon grey/black holding tanks. His prices are at least half of any other place on found on the internet. For example the 95 gallon tanks only cost $110. His name on ebay is tri-state-rock-your-world. They also make fake landscape rocks. Very nice guy, and the tanks come with fittings. My grye/black even had the probes for a level monitor.