Do a little searching over on Gun Boards in the Lee Enfield sections
Lee Enfield - 22 Caliber Forums & English Gun Pub
There were always some threads on the wire wrapped MkIII's, blank ammo, tossing tennis balls, etc.
The grenade cup itself has become hard to find. Springfield Sporters in PA used to have boxes of them for sale along with the wire wrapped MkIII's. Mostly conversions done in Ishapore, India (RFI marked).
Such it is with all surplus, one day it's cheap and it's all over the place, then all of a sudden it's rare and expensive.
Check the headspace if you're going to do some shooting with it. MilSpec was a little more generous than current SAAMI.
Many have the 'rust in bore' or the 'cord worn' mark applied. But they look awfully good when inspected.
Launching tennis balls 150 to 200 yrds isn't difficult with them. Sometimes they will just shred them right out of the launcher though so take all care when working with them.
A couple of wraps of duct tape on the ball will make it last a few trips longer down range. But don't make it too tight of a fit in the launcher cup.
Alot of fun but certainly not a toy.
I don't recall which cartridge does a better job with the tennis ball projectiles,,the parade blank or the orig grenade launcher cartridge blank. Do some homework over on those forums and you'll probably have your answer.
Seems as though a home (re)loaded blank was being used by some re-inactors too but I can't recall the components.
None of the launchers left in my group, but my favorites, a MkI* SMLE and a CLLE, both BSA commercial rifles & 2 Lee Sporters get range time quite often.
Have fun..!