I have built my own. It is easy and any decent fabricator can make you one. Some powder coat them, but just a spray with a good paint will do IMO. You don't need that much steel - I used 6" gussets and left the middles open for sumps and storage. Most are over engineered.
On a huge tank, I don't recommend leveling feet since you can literally have tens of thousands of pounds coming down onto four feet the size of a quarter (or whatever), but you will need some way to level it. I have seen the feet from a 1200G system ruin concrete over the years. I ground the concrete down a bit in places and put the whole square right on the ground. Larger feet would work out too. Of course, the steel and acrylic could bend enough to cover any small areas of not-level.