I'm wondering if anyone has seen/used
these 1.9" diam. omni wheels from McMaster. They come with a 5/16" bore, but it looks like they can be broached to 3/8" hex bore for live-axle use. I'm not 100% what they would be used for (possibly as an intake for small objects or as tiny casters), but if someone has had a good experience with them they might be something to keep in mind.
P.S. they also come in
2",
2 11/32", and
3 5/32" versions