They should be common up to 11/32 in most 1/4" drive sets. SK still makes a 19/32 and 21/32 in 1/2" drive, and Wright and Proto have 19/32, 21/32, 25/32, 29/32, and 31/32.
For 3/8" stuff Ko-ken offers 11/32 and 13/32 (and smaller), and Proto have 11/32, 17/32, and 25/32. I think 15/32 was...