As for a great place to get corncob the CORRECT grit size;;
The 20/40 grit is too small to clog/catch in primer flash holes. 40 pounds of that Grainger cob will last you a lifetime, IF you're around 25 years old. If you're much older, better give some away, or they'll be pouring it in your face when they bury you. Last I heard, it was shipped freight free. The small grit polishes faster as well.
Some pet store corn cob is WAY
too big. It'll work fine for cases as small as 9mm, but using it for .223 will result in new swear words being invented while digging that cob out of each and every .223 case neck!
Ask me how I know!