If you can make use of a rectangular tank then I would recommend plastic.
A couple of years ago I wanted to add a second fresh water tank to my houseboat. I could sling it underneath the deck, between the pontoons so length and width were pretty flexible but I didn't have much height. I found a perfect tank out of an online catalog: 115 gallons. I called the company up, fully expecting to get the brush off but was really surprised when they jumped right on it!
The company was Ronco Plastics in California. They actually charged me a lot LESS than the prices in their online catalog and I had the tank in about a week.
Check out their website:
http://www.ronco-plastics.com/ You just have to select one of their stock sizes. The tanks, as they sit in their warehouse, have no fitting on them at all. You get to say exactly where you want what fittings and they install them. So sort out what size you want, where you want the fittings and call them up.
Suggestion for you: Throw a couple of extra fitting in! Like an extra vent and extra drain. It is trivial to have the extra fittings added ahead of time, but if you decide later that you needed and extra fitting (or wanted to run the plumbing a different way) adding them later is not so easy.....