+1 for Ronco Plastics....They make TOP quality thick-walled water and waste tanks for a very reasonable price and has more than 400 shapes and sizes, over 100 of which are non-rectangular and they install fittings in the sizes and locations specified by the customer when they make the tank.
However, they're not the best source for a custom tank because all their tanks are rotomolded, made in molds that already exist. To make a custom tank would require making a mold for it, and that would drive the cost through the roof. If you can't find anything that'll work in their catalog (here's the link to their marine catalog:
Ronco Plastics marine Tanks) your best source for a custom tanks would be Triple M Plastics in Maine
Triple M Plastic Products Inc ...all the way across the country from you, but worth the shipping time and cost because their tanks are top quality.