I run 11x2.5" rear drums on the 8 3/4 in my Duster, they work great. I also run them on my Challenger, they're a good match to the later 11.75" B/R body front disk brakes.
If you have 11x2.5" brakes you can get the drums and shoes just about anywhere. I use the heavy duty finned drums, you can find them at Summit, RockAuto, pretty much any parts store should be able to get them.
Keep in mind though that 11" drums came in three varieties- 11x2", 11x2.5", and 11x3". The backing plates for each width have a different offset (because the axle flange offset was the same on all of them), so you can't interchange shoes or drums. Ie, if you have backing plates for an 11x2" brake set up you can't just buy shoes and drums for an 11x2.5" set up and bolt them on, you MUST use 11x2" shoes and drums. The 11x2" and 11x3" drums and shoes are harder to find, especially the 11x3" stuff.