First, new instruments in the student/basic range: Button Box sells 20-button Stagi Anglos in G/D as well as C/G. And Schaumanufaktur/Silvetta says that they make their 20-button models in "A-D; D-G; G-C; C - F; Bb- Eb" -- however, you may have trouble dealing with them directly if you're not in Germany. Castiglione, who sell Silvetta-made instruments in the US under their own name, offer them in D/A and G/D as well as C/G. I had a double-reeded G/D Castiglione for a while that was a pretty good instrument for a student/basic concertina.
Used 2-row 20-button German concertinas often turn up on British and US eBay in a variety of keys, though they're often sold by generalists who don't know anything about them (including how to determine what key they're in). Prices are generally low, but you may get an instrument that's not playable or worth repairing. I've generally had good luck myself, but your mileage may vary...
For vintage concertina-reeded concertinas, it's possible to find 20-button Lachenals in Bb/F. Ab/Eb, or G/D. If you're looking for one of those, Barleycorn Concertinas is a good place to start.
Are there 2 row concertinas available in keys other than C/G?
Edited by Daniel Hersh, 08 June 2009 - 06:02 PM.