Most of the eight speed screw ons I've seen have been of the MTB/touring ratio persuasion. So, yes 8 speed screw ons are relatively common, but rare if you want a reasonably small granny sprocket. Something with, say, 21 teeth on the big sprocket would be fairly hard to come by.
Just out of interest though, why would you bother? Especially when 8 speed cassettes and cassette hubs are as common as muck and can be had for pocket money. The spacing from the dropout to the bearing on an eight speed screw on setup is huge, you'll be snapping axles every ten miles.
Just out of interest though, why would you bother? Especially when 8 speed cassettes and cassette hubs are as common as muck and can be had for pocket money. The spacing from the dropout to the bearing on an eight speed screw on setup is huge, you'll be snapping axles every ten miles.