Could be wires or coil packs. I had a misfire a week or so ago, took it in a cleared the code and it hasn't happened since. Lucky for me, I suppose. My plugs are new, but the wires and coil packs are close to 50,000 miles old.
Autozone will scan a code for free, assuming that you're throwing one. That'll tell you which cylinder is misfiring. If it's just one it's likely a wire. If it's a multiple, chances are it's a coil pack (I think there's two of them? I can never remember - so 3 misfires would point to that). If they're decent employess, which the guys at mine are, they should be able to help you out (or maybe you just have to have a rack like mine for that to apply
- but they'll still want to sell you parts).