Not a stupid question at all. And I gotta say, I hate it when the wheel/brake drum combination decides to part ways with the car.

Best bet is probably to find a complete rear end from a donor car. That'll get you your axle shaft (toughest part to find)and other misc parts, from which, you can choose the best.

If you want to just replace the whole rear end, your spare must come from a wagon. If you just want to replace bits, parts from a '67 or later sedan will work. I think rear brakes are unique to a '68, so keep your current brake setup.

As for cost.. if you source all the parts that Eric mentioned individually, I'm sure you'll be looking at several hundred dollars.

Cost of a rear end? Who knows. Were you near, I'd say just come and get one off my parts car...

--
-Matt I ♥ my ♂