In Final Cut Suite, "SoundTrack Pro" comes with thousands and thousands of sound effects. There's a ton of the ones you're looking for, and top quality, too. Also check with Ljudo.com, they have free sounds to download. I've found some killers there before.
I've found that layering sound effects gives me the best results. For punches I usually use a slap for sharpness, a thud for some base, and then just a normal punch for the middle. Remember to match up the waveforms so the sounds merge successfully. Sometimes I'll throw in a swish just a little bit before the impact.