Best bait? What's handy? Ideally, catch your bait out of the same waters your fish. A long time ago, another member wrote this, Catch a green sunfish/bluegill cut off it's head, skin the body, cut off the fins. Hook the head on one line, weightless (in a lake/pond), & hook the skinned body on another line, weightless. If you're not bit in 20 minutes move and rebait. It's durable, and it catches fish. Like several people said, shad are great, use a cast net to get them. Depending on the size of shad, try the head on one line, guts on another, and if you can have 3, the rest of the body on the last.
I've seen channel cats being caught on liver at a local park, like kids can catch bluegills off a bed, and these were all 2-4 pound eaters. I returned the following week and did nearly as well and added white bass to the mix as well.
I've caught cats on crankbaits, spinnerbaits, huge bass jigs, tiny crappie jigs and even top water poppers.
Freshwater clams make a good bait as well as leeches and worms.
In my experience, you can spend money for cat baits, but it's probably just as easy to get your bait from the water you're going to fish.