When I built up my 3-speed, I used a 38-tooth chainring with a 19-tooth sprocket, giving a 2:1 ratio. You should be able to get sprockets up to 22T, so you could get an equivalent ratio with a chainring up to 44-tooth. SJS cycles are a good source of sturmey-archer parts. If you really want an odd-size sprocket, it's possible to take apart a cassette and slightly modify one of the sprockets to fit. Failing that, you can adjust the gearing by changing the chainring size.