I have never paid more than $19 for a PC-FX pad. I see them regularly for $30, which I think is too much, but , y'know, if it's mint and in box, maybe it's worth it.
The PC-FX looks so sweet, I'd hate to see it with anything other than the original pad, although your skill must be respect-worthy for even wanting to try it. Good luck.
You could try SuperFamiShop... they have pads sometimes. I've ordered from them now, and recieved my goods, so I trust them.
Here's the URL:
http://www.superfamishop.comAs of today, they have no pads, but keep an eye out, or start your Frankenstein-ing.