We did a sanitation-system replacement project recently, following Peggie's advice to source from Ronco if they had a viable size or from Triple M if we needed a special design.
Turns out we found a good 40-gallon option from Ronco, but we could have gotten a tank maybe 7- or 8-gallons larger from Triple M -- a much more complex design, not a cuboid rectangle -- but the cost would have been about double.
I thought the Triple M estimate was quite reasonable for the outcome we envisioned, but then we decided we just didn't need to push the envelope that much to achieve our basic goals. The Triple M lady was very easy to deal with, though, very fast estimate, would have been fast delivery, etc.
Finding tank sizes on the Ronco site sorta sucked; no decent search tool. It was easier to search for Ronco (and some other makers) products on plasticwatertanks.com or plastic-mart.com. Once we found a good option, dealing with Ronco direct was relatively easy.
South River, Chesapeake Bay