I know I have seen it offered somewhere. It may have been from Myron
Toback in NYC. They have a huge selection of tubing. If they don't
have it they can make it. The prices are fairly high. Call them at
800 442 8444 or 212 869 0808. Be forewarned they can be kind of rude
on the phone. Were you looking for silver or gold? In gold they have
a 25 dwt. minimum for custom orders.
The other thought is Richard Mawdsley, the king of all tubing. His
email is email@example.com. He is an artist not a supplier, but
he works with tons of different tubing and may know other sources
for odd stuff.
You may also be able to draw square tubing to rectangular if you
have a draw bench. You will most likely need an annealed copper core
with some graphite on it or oil, that is the size and shape of the
inside of the tubing you want to end up with to do this well. Leave
the copper sticking out of both ends of the tubing when you draw it
down and then when you get to the shape you want grab the copper at
both ends and stretch it to reduce its cross section and it should
slide out. Don't do more than 6 inches at a time to reduce the
chance of getting the copper stuck.