Since you don't plan to reload, surplus ammo would be my choice if you can find it. Check the local places (Craigslist, OfferUp, etc.) for either Lake City or HXP (Greek) surplus ammo. You might also find some leads on ammo at the local (if you have any) CMP/NRA service rifle matches. If you still can't find any, I'd buy the ammo from CMP or Creedmoor Sports. Be careful using regular 'ol .30-06 from the store, as it's usually hotter than what the Garand operating rod can handle over the long term. That, or buy an adjustable gas plug and shoot whatever you like.
As for clips (en-blocs), it's a personal thing. I've got hundreds, but I wouldn't keep less than 8 or 10 for a casual plinker. Also, the manufacturer of the en-bloc matters a little bit. I've had issues with foreign manufacturers' clips and stick with USGI stuff. $2 each for USGI clips is pretty much the going rate, and anything less than that is a good deal. I like SA clips when I can find them (and they're not hard to find).
Also, like any other hobby, people collect the clips. Each manufacturer stamps either their name, initials or other identifying mark on the bottom of the clip in the little circle. Certain manufacturers command a premium with collectors. Find any that are marked "WRA" (Winchester) or have cut notches on each side (vs. stamped) are easily worth $50 each on eBay.
Also, if you're into the historical aspect, the book, "The M1 Garand Rifle" by Bruce Canfield will tell you more than you ever needed to know.