The .45 ACP is a popular round, used in pistols like the Colt M1911A1, Heckler & Koch Mark 23, Heckler & Koch USP, Glock 30, Glock 36 and more. It is also used in submachine guns like Heckler & Koch UMP45, MAC-10, Thompson Submachine Gun and more. While not as plentiful as the 9×19mm parabellum, you should still be able to find it in gun stores, police stations and military bases (though it might be harder to find them in American military bases nowadays, as the U.S. military replaced the M1911A1 with the Beretta M9 in 1990. The M9 is chambered in 9x19mm parabellum).
As it is used by both submachine guns and pistols, you can share ammunition between your primary (if it is a submachine gun) and your secondary weapon, if both use this cartridge. Because the .45 ACP is a subsonic round in its most common loading, firearms chambered for the .45 ACP cartridge can be completely silenced. The .45 ACP produces more tissue damage than the 9×19mm parabellum, but is bigger and allows less space for magazine capacity. Against zombies, where only headshots matter, the extra power provided by the .45 ACP over the 9×19mm would not be very useful. While not a heavy round, it is larger than the 9x19mm, meaning you might not be able to carry as much ammo. It is also more expensive than the 9×19mm.