I would carry a Glock 17/19 in a IWB holster with two spare mags in a carrier. 9mm can be found here and all over the world. 9mm is a NATO cartridge, and used by the U.S. Military. In hard times, 9mm will most likely be the ammo one will come across.
In not so hard times, 9mm is cheap to buy, can be found in every gun shop/ sporting store (when there isn't a shortage), and is easy to reload. The 9mm is accurate and can penetrate adequately. The 9mm caliber also lends itself to high capacity pistol designs , like the Glock 17/19. I chose Glock because they are IMO/IME, simple in design, reliable, durable, accurate enough to get the job done, highly rust resistant, low maintainence, and not ammo sensitive.