There are many options.
1) Go crabbing. Get your fishing license and set out some crab traps! If you've got a boat, or a friend with a boat, this should be easy. If not, consider getting a free boat.
2) Buy a fresh crab. Your local farmers' market might have a fresh crab stand; I got mine at the Oakland Grand Lake market. Seafood shops, yuppie grocers, and asian grocery stores may also carry fresh crab.
3) Buy a pre-cooked crab. This is sub-optimal, but will certainly do if you really can't find a live source or don't want to deal with live pinchers. In any case proceed to step 4 to clean your crab.
Note that a happy crab will be active and feisty when disturbed. Pick him* up with tongs or carefully with your hands, making sure to grab at the back of the shell. The crab can reach pretty far back under his body to pinch, but your upper fingers are quite safe. You can also grab hold of a couple of back legs on either side to immobilize your crab, but be careful not to break them off.
*you're only allowed to keep and eat the males