Sprocket Specialists makes custom ones down to ,I believe, 38.
Chris products has a 36/37, I think. CBX can verify this one.
and Rebel Gears also has custom sprockets. Never used one on my 500, so I'm not sure on sizes.
I've used Sprocket Specialist for over 10 yrs with great results, and have heard good things on the other two places I've also mentioned.