Your best (and probably only) bet is to contact the manufacturer and hope that they have an upgrade. Generally, the most you can hope for is to get replacement drivers of the same kind.
As for your chances of success when undertaking an upgrade yourself...it depends on how critical a listener you are.
A friend of mine had an old pair of Dahlquist DQ-10s with dead woofers. (No surprise there--they had foam surrounds and the foam rotted.) He went to Radio Shack and bought a pair of woofers that fitted into the holes. He listened to them for a bit...then found some sucker who would buy the poor mongrels off of him and went and got a new pair of speakers.
The likelihood of finding drivers (particularly the woofer, since it must match not only the crossover, but the cabinet as well) that will simply drop in and function well is virtually nil. By the time you've bought new drivers, modified the crossover, and possibly even the cabinet, you've invested enough effort that you might as well have just started from scratch.
On the other hand, if you just want a party speaker, then maybe you can achieve happiness with whatever comes to hand. Like I said--it depends on your goals.
OK, I'll admit that I took a slightly different route, but drop on over to AudioKarma and look into the Infinity Speaker forum. Look for RS-3000 rebuild thread. It will take you through a speaker rebuild and upgrade process.
I have hot-rodded a lot of speakers. It's a sort of part time hobby. Not too hard. It depends on knowing the Thiel/Small parameters of the driver you are replacing? Maybe the cross-over components, what consitutes an upgrade, etc.
Bracing cabinets, changing cross-over caps to better grade, altering the crossover points or curves, adding or changing stuffing, changing drivers, internal wiring to larger gage, changing cheapie spring plates to binding posts, bi-wiring or bi-amping, changing to better feed wires, adding stands, deadening the room, all are upgrades that are not to hard to do.
Please don't trash a set of Dalquists. If you don't want them, PM me and we'll see if someone on another forum wants or needs them for parts at least !!!!