You can sometimes find components in the
shotgun news to "Roll your own",
But I think the "concussion shells"
(What Uncle Sam calls the bird bombs)
Are now strictly regulated.
The only thing I have seen recently in
37 mm is Smoke and flares, and they
Sell out very quickly. You may find some
"whisler" shells every now and then but I have
not seen them in a couple of years.
(plastic Ball with slots cut in it, Makes an
aweful SCREECH when you fire it).
Prospects for ammo are not much better with
26 mm either.