Try computer repair shops, the junkyard, or anyplace that accepts/dismantles/works on old computers. I used to work near a place that accepted donations of old computers, dismantled 'em, and used the best parts to build machines for poor people. They threw the parts they didn't use in the same dumpster my store used- seems like all of the computers I pulled out of there had those cheapo soundcards, and sometimes the dedicated gameport cards as well. It seems like every computer repair shop I've ever been in has boxes of old parts sitting in the back that they've pulled from scrapped systems. I have also rescued computers from the dump, they usually have a big pile of 'em sitting in the hazmat/recycling area up front.
I don't see why you have to bother trying to disable everything but the gameport on it, either. Just don't connect anything to the speakers, mic, or line inputs on the card, and who cares whether they're working or not?