Which caliber? Does where you get ammo come factor in?

I was thinking of getting a Marlin Trapper SBL in .357 but after a trip to my local Walmart I might be rethinking it. I forgot that Walmart stopped selling hand gun ammunition and only sells rifle and shotgun ammo (with no .223/5.56.) so what is the groups opinion? Go with a traditional cartridge .45-70/30-30 or stay with the .357?