New guy here so my opinion may not be worth much
Now 37mm are basically unreglated (at least federally) as long as you fire only flares, smoke, gas rounds (basically anything that's not 'antipersonnel' according to the ATF).
There are launchers that mount under an ar-15 that look like an M203, and other break-barrel launchers that look more like an M79. There are even some pistols. There are some professional launchers that are used by police.
The commercial rounds are mostly smoke or flares, but are very expensive... Most people reload. A lot of people fire 35mm camera film cases filled with sand or chalk.
You can register your 37mm as a DD (Destructive device) and fire shotgun shells, adapters that fire multiple .22 rounds, etc.
You can buy a 40mm launcher (m203, M79) but most of the rounds that are commercially availalable are practice (chalk) rounds surplus from US military.
There's much more to it, but that's it in a nutshell.