I've been looking all over the net and it seems very hard to find subwoofers that are not powered. I have a nice NHT subwoofer amp available to me and I'd like to find a subwoofer to use with it.

I can't afford a very expensive speaker and all I see on ebay right now is a beat up 8inch NHT. Even subwoofer kits are all powered as far as I can tell.

Any suggestions?

Sure; the SVS unpowered cylinders are available if they fit your budget.