When I was looking for a tank to replace my leaky holding tank Peggie Hall recommended Ronco Plastics. I went through their catalog and found a tank that fit but I ended up going with a Plastic-Mart tank because they looked to be built exactly the same way and were cheaper. The Plastic-Mart tank was bang on the dimensions and the fittings are good quality. After 5 years I've had no problems with it.

Plastic-Mart has a web page specifically for marine water tanks FWIW http://www.plastic-mart.com/category/30/marine-water-tanks