Try Lanches in Annemasse - they're reopened now for the season, and offer a boot fitting service. 

http://www.lanches.fr/index.php?page=english

Have you been to the big ones - Athleticum, Ochsner?

ochsner had nothing in aubonne, signy or geneva.  where is athleticum?

Give "Ski 137" in Versoix a call to see if they stock your size... if so they have great service and top specialist brands for both ski and board.

Alternatively, if you have a car, it could be worth calling some ski shops in some larger resorts (Verbier, Nendaz etc - I mention 4 valleys since there is a large snowboard following who ski this domain, plus it hosts the X Games ) and seeing if they stock your size beyond beginner boots. Most of the shops advertise on the resorts websites. Often theyre not that much more expensive than Geneva shops, and it could be a nice afternoon trip on a Saturday.

checked out ski 137 this arvo - 1 boot in the whole shop in my size, so an option :)

will check out the place in annemasse on the weekend...