I ordered my tank from Amazon (well, an Amazon vendor), and it was less than $100 shipped. As I recall, the tank itself was only about $58, but shipping was another $20, or so. There were a variety of sizes and shapes, though I don't recall if any of them were as slim as what you're looking for. Mine is about 8" high and holds 21 gallons. It's a great fit for under my trailer, but, apparently, I have a bit more room to spare than you do. It might be worth checking out.