Look at this site. There are others. They have a Kaanar(sp?) blooper for $7500. LMT 203's for $1995, seen 'em for just over $1000 used before. Sometimes a 302 or HK or FN launcher comes up but rarely and they cost a lot. There is a 6 round 37mm for $1895. M26 shotguns, Serbu's, etc. They have it all, and it rotates a lot. Lots of MG's and hosts for those of you in those states. DD's, SBR's and AOW's for the rest of us. BTW, an AOW attached to a rifle is STILL an AOW, not an SBR. So a 203 or even a masterkey is still an AOW. I called ATF and checked a while back, though never got one. Not sure about attaching it to a dedicated stock like the KAC stock, you'd have to call, but I can see it going both ways. Bottom line was they said it can only be registered one way and an AOW isn't an SBS. I'd probably want it in writing though were I to get one.
There are more rounds available for the 40mm than 37mm actually and they carry those too. Beehives and buckshot can be fun, but shotguns have far more power. Their stock varies, if interested you need to check it often, they sell fast. You can't get the HE or HEDP rounds even if you pay the tax --I know of nobody that will sell military explosives even if you have the licenses or pay the taxes. Trust me, I called. I was interested in one, but unless I could get the HE rounds, I wasn't interested. I have a safe place to shoot 'em and know I'd have to pay tax on 'em so use would be extremely rare and I'd shoot mostly TP's, but still. In short, it's a fun toy but just a toy without the HE rounds. Even flares and smoke are better deployed using other methods.
Ebay sells IR lasers and 203 sights for pretty cheap.
IF you find anyone that will sell HE rounds or other military explosive rounds regardless of caliber LEGALLY as a DD on a Form 4 or whatever, let me know. No illegal or "iffy" shit though. And don't think of making it yourself, it's not worth it and you'll probably blow yourself up; a lot went into designing those grenades. That said, there are loading kits for 'em, just don't load shit that goes "BOOM!"
Good luck, and have fun with the TP round, remember they go brittle with time so try and get newer ones and use 'em up sooner than later. Be easy on 'em. Superglue can fix cracks also, some use scotch tape. Weight of lead base means it won't affect trajectory enough to even notice. Try and buy in bulk, ammo can be kinda rare.