Can anyone tell me where I can buy rectangle exhaust tips like those on the Silent Thunder exhausts?

Installing through transom exhaust on a boat and due to space restrictions can use round ones.

I think you'll have to have them custom made.
Even with the Silent Thunder system the thru-hulls are still round. You'll need custom adapters to go from the round thru-hulls to mate with the square tips.
The square ones are just screwed to the fiberglass water boxes (mufflers) on the Silent Thunder platform.

Exactly Iggy. The tips on the Silent Thunder box do not connect to anything and just finsh the exhaust outlet off.

I bet one of the west coast custom stainless poeple can help you. I will try and look at my Eddie Marine, Rex Marine and Stainless Marine catalogs and see if they have something. They are all on the web also if you want to try them.

CORSA makes a million different kinds of exhaust outlets for a lot of the maunfacturers. Try them at: www.corsaperf.com