No offense, but I still have a hard time calling those things they sell in a) grocery stores or b) all you can eat places, "sushi". Those things are nasty. They use cheap, low grade fish, and the rice is all fscked. ugh.
If you can't go to Japan, come to Sunnyvale and be ready to spend $150 for a meal. I'll get you real sushi .... ;)
Update: Hmmm, Interesting. I lose reps for this?
Update: Correction, grocery stores in US. Mostly, the ones I know first hand are grocery stores in MidWest in general, Dallas, and the Bay Area (yep, don't like the ones here, either). If you get a sushi thing from a grocery store in Japan, it's definitely doable depending on the store... ;)