Am I the only one that's having trouble finding this "common" ammo?

Maybe :D

You can get it from Killian Darkwater, from some Raiders in random encounters and caravans, from Jacob in Oldtown (The Hub), from the Gun Runners, from shelves/crates/lockers on various maps, from other random encounters as "free items" (on corpses and such).

Killian only seems to sell ammo for the Deagle and raiders seem to use shotguns.

Did you scroll through his entire inventory list? In barter he should definitely have 10mm ammos.

If not, try clicking the "Stuff" on his tables and see if that changes his barter offerings.

Well it looks like I missed a table last time I went to his store.

That said it isn't much ammo, but it will do.