I picked up some sushi for a meeting we had at work today....so we drove over to Fujiya on Clark at Venables. Fujiya is a great place to get boxed sushi, sashimi grade fish (Angel Seafoods just up the road is actually best for this), and other Japanese groceries. You can also get bento, and cooked foods to go.
All the sushi is freshly made in-house in the back. The best time to pick up sushi is just before lunch time and dinner time when you can literally grab the boxes just as they finish making them. The rice and fish is guaranteed to be fresh at these times.
So for those times when you need to get sushi to go, this is the best place in Vancouver to get it, IMO.