I agree they are hard to find. If you have access to USA based Mouser or Digi-Key there are a few choices.
For an encoder with a 6mm D shaft:
Mouser #450-BA600 (Eagle Knobs & Dials # BA600) black, no indicator, 7/16" dia with a 5/8" skirt
Mouser also has knobs for encoders with a fluted shaft but I don't have numbers handy.
For an encoder with a 1/4" D shaft:
Digi-Key #GH7339-ND, black, no indicator, about 5/8" dia
Digi-Key #GH7342.ND, black, no indicator, spinner, 1.3" dia, about 1/4" tall
Digi-Key #11K5020-KCAG-ND, gray, no indicator, spinner, 1.3" dia, about 5/8" tall
Good luck,
Chip