Specifically, I would like to know on which maps the Banshee is available, but a list of the maps and the vehicles available on each would be ideal. This is for the PC version.
Banshees are available on Blood Gulch, Death Island, Gephyrophobia, Infinity, and Sidewinder.
Here is a list of maps and the vehicles found therein:
Some of these I could remember off the top of my head, the rest I consulted this list of multiplayer maps. I tried to include the differences in the PC version of the game, but corrections are welcome as I was primarily a Xbox player :)