.30 Carbine ammo
Sixer--First of all, congratulations that your Grandfather trusted you to carry on the family tradition. The M1 Carbine is a neat, neat little rifle!
I recently got one from CMP, and while not "family" like yours, it sure is a handy, light firearm. The designers did it right: Light and handy, and more accurate and easier to aim than the .45 ACP pistol.
As to ammo, I ordered some from Cheaper Than Dirt, which came on stripper clips, which are very handy for feeding those magazines. You DO need a stripper clip feeder to use 'em though. Price IIRC was pretty good too.
Wal-Mart is of course an easy source, but spendy, the way the carbine--or any autoloader--eats ammo.
Bo't some reloads @ a gun sho, which hadn't been sized properly, and which don't allow the bolt to close quite shut, w/o a hearty slap on the op handle. Makes quick 2nd shots impossible; annoying; I won't be buying any more of that reloaders' product.
Bo't other reloads @ another gun sho which work just fine. The resizing of the cases, all the way down the body, is crucial.
Of course, my cheapest ammo, and best, all comes from Smokey Joe's Reload Shop. Currently I'm working at getting good at reloading .30 Carbine, and it is different from any other cartridge. I've not mastered it yet; all I can say is I'm making progress.
BTW, my sizing die does not contain the decapping rod (that's in the bell-mouth die) so I resized the live ammo that didn't fit, and afterwards it almost fit but still not perfectly.
Anyhow, what I'm getting at is, that this is a perfect excuse to get yourself into reloading.