So if I wanted to buy an M1 Garand, would I be better off getting it in a .308 just because I could get modern ammo for it?
Only if you want one in .308.
Federal (and other companies, I believe) make Garand-specific loads. There is also a list floating out there of other commercial ammo that is Garand-safe. I believe plain old Remington 150-grain Core-Loct is on that list, if you want a good commercial hunting load.
Also, changing out the gas plug with an inexpensive aftermarket plug (by Schuster, McCann, Garand Gear, etc.) allows shooting of pretty much any modern commercial ammo through a Garand. It's a drop-in part. Some need adjustment. Some don't. It does not permanently modify the gun, so you can put the USGI plug back in for matches that require it.