I put them in myself, and I have them made at local sheet metal shop out of aluminum. I use 1/8" aluminum my guy cuts them, brakes them, and welds them for about $200 ea. You can buy them cheaper than that but the narrowest ones I have found are 15" wide. If you look under your trailer you probley have I beams placed at 12" spacing, to use the wider tanks you have to knock(cut) one of the I beams out, Store bought conversion places do it all the time. I'm not a big fan of 24" spans of aluminum floor with no supprt under it. So I pay a little more I get tanks that fit the trailer I am building, and I get them out of aluminum, I have had people drag the valves off(easy fix) but have never had anyone rip a hole in one(dents, no holes).

Search around the net. I've seen them out there.

One other thing you should think about is getting two aluminum I beams and mounting them under the running boards. I can and have sunk into variouse material without ever breaking a pipe or a tank.