gjw,
Load it yourself. I have a lot of fun shooting carpenter bees with a .410 pistol, using 444 Marlin cases loaded with used polishing media( could just as well use grits). I cut wads from meat trays and seal the overshot wad with carpenters glue. There is no good reason you couldn't use small loads of fine shot. You can load them without much equipment. If your gun has a 2" chamber, you would need to trim about 1/4" from the length.
Mike