I dont have access to my 1100, or an 11-87 at the moment, but IIRC, the magazine tube is pretty much the same as the 870's , and the same extension should work.
Don't know of a place to get a barrel offhand, but I'll look around.
I've heard of people just "hammering" them out with a socket. Put socket in, tap a few times, wiggle, tap a few times - until the dimples bulge back out.