If you just want the tank for compliance - rubber blade tank works fine (but don't try to actually use it
There are also a few manufacturers that have large catalogs of tanks they make. You could see if one of the shapes and sizes fits.
Here is one: http://www.ronco-plastics.net/marinetanks.html
Raritan has a catalog too and so do a few others, I don't have a link for right this second.
For my boat of same vintage, none of these tanks provided optimal size so I went the custom route. It cost me a pretty penny, but I can go for weeks without pumping out.