I bought a 90/90-21 today - but in Croydon, UK.
But the coincidence persuades me to share this: about 13 years ago I did a ride that went through Kiev and Moscow (and St Pete), and wherever we stopped, (on UK-registered bikes) to ask the way or at a cafe or for petrol, a big crowd always gathered. In the big cities that would include local bikers who were always very helpful, so you may find a source of tyres that way.
Maybe you've already experienced this.
Amongst other things, we received free accommodation, repair-shop recommendations, use of workshop, and secure parking info through these gatherings in various places.
In Kiev, the crowd grew pretty big because we were lost and everyone wanted desperately to help. Eventually the police turned up, looked at the address we were looking for, and escorted us all the way there in their police Land Cruiser. Complete with formal salute when they departed, job done!
Good luck with the tyres!