The question is probably more like "where CAN'T I find Sushi in downtown Vancouver?"!!
There's tonnes of places you can try, but if "Downtown" is what you need then you best head off along Robson Street where you will find places like "Gyoza King" (recommend), "Tsunami Sushi" or "Guu Sushi".
If you're a true sushi connoisseur, however, you'll want to leave the Downtown core, head over the Granville Street Bridge and head along West Broadway to "Tojo's". about 8 years ago