I don't have his contact info handy, but Lee Stofer (who posts here fairly often) carries a limited selection, and is a first-rate person to do business with.
If Lee doesn't have what you're looking for, the Baltimore Brass Co., linked in the 'shops' section of this website, has a sterling reputation. When spending $4000 on an item like a tuba, making a two-or-three-day trip to Baltimore to get the right instrument seems worthwhile (to me, anyway). His website shows his used instrument inventory and which new instruments he carries. A long day-trip beyond that, but also of excellent reputation, is Dillons (again, linked, also showing inventory...). Of course, call ahead to make sure there will be someone available to assist you the day you plan to be there.
Conferences are another possibility. SERTEC (SouthEast Regional Tuba/Euphonium Conf) is upcoming in Athens, GA, later this month. See
http://www.arches.uga.edu/~dzerkel/sertec/
I'd imagine some of the vendors would be there. You could contact them to find out what they're bringing, and possibly request they bring any particular instrument they have in stock. I don't know what school conferences might be upcoming where tuba vendors might be showing, but that's another shopping option.
Be advised that there is an establishment or two out there who are not highly regarded within the Tubenet community. In particular, if you have not seen enthusiastic praise here for an establishment, tread cautiously. While good equipment at a good price CAN be obtained from these vendors, you could also easily wind up either paying too much, or getting a horn that's ill-suited to you, or both.
Best of luck!
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Joe Baker, who suggests first learning who will be at SERTEC.