Just to add, steel is not necessary as bracing for an aquarium. The silicone used to glue the tank together, is more than adequate to hold it together. However, I find it odd that no retailer will order a tank of the size you want. It is usually cheaper to buy a prebuilt tank with warranty, than to buy the glass and do it yourself, or have it custom built. I have built a number of tanks, but only because the glass was free.
Join a fish club. Don't have one in your area? Start one.