Jesse,
Congrats on the Forte's. Very cool sister, too.
It wouldn't hurt to call Klipsch parts, 800 number somewhere on the main Klipsch.com links. They can tell you what it would cost for replacements. Not too likely that they would have the original woofer still in stock, but they can probably recommend an appropriate substitute.
There are other forum members with the knowledge to make suggestions to locate complementary substitutes. Might even be that the odd woofer would be OK with a mate of the same make.
You should be able to get the right woofers in there and not spend a lot of money.
The very cool looking things on the back that look like woofers are actually "passive radiators". It is clever engineering used in some speakers as an alternative to having bass ports on the cabs. They PR's are designed to have the proper mass, stiffness, and other characteristics so that they are finely tuned to let just the right amount of bass into the room.
"I know I'm paranoid.... but, am I paranoid enough?" --Tom Clancy