Yeah, all that’s available is:
1/8",3/16",1/4" and 4 mm, 6mm, 8mm
and the aforementioned 3/8" collet. Also, due to their design, they have a narrower clamping range than ER-style collets do.
FWIW, I’ve been meaning to look into a 3/16" and 8mm collet — anyone have some suggestions for fabulous endmills which would make that worthwhile?