You should be able to get it as a special order from a
Martin or
Tsubaki reseller. It's usually listed as a "double single" sprocket. Alternatively, it's not hard to make something broadly equivalent, given a few machine tools.
Edit: I looked at Tsubaki's
catalogue, and found
this, which suggests that they only have double single sprockets in #40 and above. And similarly, in
Martin's catalogue of stock sprockets, the double single appears to come in #40 and above only.