Spent tubes are/were sold by military surplus vendors. They have been turned in at gun buybacks (For a profit. Many police don't know that once fired, they are harmless and useless.)
Couple of VERY rich collectors have RPGs with warheads.....RPG-7 + half a dozen HE rockets =$50,000 in Destructive Device taxes, ammo magazine construction, Federal Explosive Licenses, Type 10/02 FFL SOT, ITAR registration, etc. Weird that you “CAN” get an RPG...but not 40mm HEDP. Launchers, though, are plentiful. I form 1’d my M203 receiver and tube. Chalk round box around 300/case.