Fulton Armory, Norm Ricci, perhaps some others over at Culver Shooting Pages at
www.jouster.com
Beware of off-spec or refinished parts. I have gotten nipped in years past. And the prices are up, unfortunately.
I have a "couple" of Garands and they are quite trouble free once you pick up 50 years or so of slack on new aquisitions. I think it's a good idea to simply replace all springs on a "new" rifle so as to have a known starting point on things. The op rod spring in particular, which should measure out at about 19 1/2", and be replaced every thousand or two rounds. Just get a standard weight one, not an extra strength one. I have also replaced clip latch springs and here a HD one may help.
Small bolt parts are good to have/replace--firing pin, ejector & spring, extractor and spring.
The books list allowable wear on op rods and cylinders. They do wear, and new ones are getting pricey. Haven't had an op rod rebuilt but probably will have one done on principle. Not sure if anyone is rebuilding gas cylinders at this time.
Thus far I have just bitten the bullet and bought new ones, while wishing I had laid some in years ago when I first thought of it (but was broke). My NM rifle wears new ones and doesn't get shot too much. My usual shooter Garand parts are in nice shape--but I have a new op rod and cylinder so my kids and grandkids will be able to shoot it years from now.
Barrels are the easiest things to come by and ought to be around for a long, long time to come.
Always bear in mind that a Garand is a work in progress as well as a piece of history.
It don't mean a thing if it ain't got that ping.