https://www.gunbroker.com/All/search?Keywords=m79
https://www.firequest.com/37mm-accessories.html
The launcher is not restricted. Likewise, training, smoke and flare type ammo is not. Anything that goes boom is considered a destructive device (DD) so that ammo is restricted. DD means propellant charge greater than 4 oz and/or explosive charge greater than 1/4 oz. See Title 26 section 5845 subsection F for details.
As always, consult ATF, state and local laws, and get the advice of an attorney prior to attempting a purchase.
A fella ought to have a couple of those for crowd control when the Zombies go full AOC.