Two points to be made. First, the answer to this depends to a very large extent on where you are going. If you are in South Africa, there are usually stores nearby which can supply most of the common calibers, and some uncommon ones from time to time. You might not get a brand you recognize, you might not get the bullet weight you'd prefer, and you'll surely pay a whole lot more than you would in North America, but you will find a pretty good selection not too far away. If you are going to Cameroon, and Air France loses your ammo (as it did to me), they you're pretty limited in what you can get. Thankfully, I had a .375 H&H, and again, thankfully (sort of) you don't usually get to shoot a lot in the Cameroon jungle (three shots in 14 days!), so the half dozen bullets we were able to scrounge up more than did the trick.
Second point, and back to the original question. I think most of us know what common calibers are, and which ones are more arcane. Personally, I use a ,300 Win Mag for plains game, and I never ever worry about not being able to find ammo. For bigger game I use either a .375 H&H, a .404 Jeffery or a .416 Rigby. I don't ever worry about the first, and worry more about the latter two.
