Well, it's the specific teeth number you want that might be a problem for people to reply to...
I saw your post about 1/2 hour after you posted it, but, I didn't know about the specific number of teeth, so I didn't reply.
As for sprockets in general, bikebandit.com, denniskirk.com, jcwhitney.com, sprocketspecialties.com (or ...specialists.com??), ebay, or if all else fails, Google "Yamaha RD400 sprockets"
But, of course, that avatar covers over a multitude of sins....