The absolute GREATEST, cheapest and best assorted place you will actually find in neighbouring France, at Chinese and Japanese grocery store MOMO's, just on the other side of the Veyrier border (Salève side)  - turn left immediately after the border crossing and then just stay on the French Route Nationale heading towards Etrembières. Momo's is immedately to your left after the roundabout, circa 500 meters after you've crossed the border. You'll see it, adjoining restaurant (also really nice ) and large parking in front from the route. They have everething you can possibly require; fresh, canned, tinned, dried, frozen.... You name it. They have it!                                                          

Ps. Frech vegges come in on Thursdays, in case you will wish to also make some seaweed salads, bok choy, bean sprouts or other side dishes to go with your sushi.

Bon appétit! -  Eva  

Thanks for the response! I will try that supermarket next time but I won't have time to make it to France this week, any suggestions for Geneva?

You can get Sushi rice, rice vinegar, sheets of seaweed for rolling, ginger etc (all "Saitaku" brand) in Coop, on the foreign food stand. Definitely in the Servette branch, probably others too.  I'd guess you can't get all the stuff Eva mentioned but they even sell salmon prepared for sushi.

Bon ap!