I bought a large tank from this outfit a few years ago:
http://www.ronco-plastics.com/ I needed a fresh water tank for my houseboat. Because I was going to sling it under the boat between the pontoons it couldn't be very tall but needed to be long and wide. I ended up buying a tank that was ~8 feet long by 3 feet wide by 9 inches tall. Their catalog listed it as costing $360 but when I actually called up to and ordered it was a lot less, like $280 including the frieght to ship it.
They were great to work with and I got it really fast. They make the basic tank without any fittings and then you specify what fitting you want and where you want it. I just made a sketch and faxed it to them.
I recommend throwing in a few extra fittings. It doesn't cost much to have them add a fitting but it will be a whole lot harder to add one later!