Semi tires are ok for dragging and hitting, but too light for flipping. You want a tire off a Front Loader or some big friggin earth moving machine. My tire stands about 5", weighs maybe 200-225 and I'm ready for a bigger one.
Find a large tire shop that works on big stuff, they'll give you one or charge just a few bucks for the casing.
Say you're doing it for a Youth Fitness program or Strong Man comp, and they'll bend over to help ya.
I have an 8lb sledge, works good, guess you could start with a 10 as your such a beast.
Just don't let the damn thing bounce back off the tire and whack ya. You'll be in some serious hurt. Try not to smash any other body parts as you are now moving into some funky stuff. While you're out trolling for tires, stop into a beer distributor, buy an old dented up keg. Add water-more fun then reading the "Starting" thread on the CF boards.