Dometic/SeaLand 975MSD Sanipottie is the only one still made in the US. Available in both 2.5 gal and 5 gal model. I strongly recommend that you go with the 5 gal version for a couple of reasons: 1. It's "household" height...the 2.5 gal is like squatting on a bucket. 2. It holds 50-60 flushes..the 2.5 only about 25...hardly worth the aggravation and, in some areas the expense, to pumpout. I'm pretty sure the sbo.com online store has it in both sizes...get a quote from Dave if it doesn't show up in a search for it.
The 5 gal tank weighs about 50 lbs when full, which makes it a bit heavy to carry into the cockpit to dump over the side, and besides disconnecting and reconnecting pumpout and vent lines a lot of times--which you'd also have to do to dump the small one--isn't a great idea. The best solution if you want to be able to dump at sea would be a y-vave and macerator pump or manual diaphragm pump in the pumpout line...this would require a below-waterline thru-hull.
--Peggie