You can find one-row diatonic accordions in C, G, D, and less often, A, and even more rarely, flat keys or keys with 4 or more sharps.
If your dream is to play Quebecois music, a one-row/four-stop button accordion in D is a good choice. The stops turn banks of reeds on and off, and the tuning will be in what’s called equal temperament. This is what you hear when you play a scale on a piano, assuming the piano is in tune.
In the Irish music world, one-row button accordions are typically called melodeons. They, too, will be in equal temperament.
For Cajun music, a one-row/four-stop button accordion in the key of C with the 3rds and 7ths of the scale tuned slightly flat is pretty standard. Cajun one-rows typically have little or no tremolo. Not all one-rows in C are Cajun-tuned, so if you’re shopping for one or the other, ask or listen to the scale.
Acadian one-row, four-stop button accordion in D
Hohner Presswood button accordion in G
Introduction to Concertinas and Accordions 