Actually you can just cut off the extra signal wires. Or at least I know it works with some cards, because I've seen it done successfully before.
You might check and see if Matrox or some other manufacturers have x1 cards available. I know Matrox cards can be kind of expensive, though. If you don't mind doing the cutting, grabbing a cheap ATI or nVidia card would probably be your best bet.
edit: Looks like there are a couple ATI x1 cards on NewEgg. The prices are ridiculous, though. Both are HD4350, and they're like 3x more expensive than their PCI-E x16 counterparts. :/
Definitely looks like buying a cheapie and cutting the connector is your best bet.