A USB converter would be the best bet; StepMania's site actually lists ones that are most compatible with dance pads.

As for pads themselves, if you're willing to put some time into it, you can't beat building your own. I built the actual base to mine two summers ago and, despite issues I've had with poor soldering jobs in the wiring, it works fantastic. It only came up to be about $40 out-of-pocket expenses for me, too, since a lot of the supplies were things my dad had out in his shop - it might not work out of everyone this way, but if you know someone who is in to carpentry, it's worth a shot

Red Octane has a reputation for some beasty pads, too. I think opting for a thick foam pad is a good idea for anyone somewhat serious about playing, since they're not as expensive as hard pads but have much better performance than the super-thin pads.

Which is not to say I'm a terribly serious DDR player in building my own; just bored and curious as to what kind of results I would get