US customs regulations allow you to bring back one litre of spirits duty-free, once you have spent 48 hours in Canada. You can actually bring back as much as you want, as long as you are prepared to pay duty, which I believe is 3% of the value. Usually the customs officers simply don't want to bother with this, and wave you through, as long as they think you are being straightforward.
Lawrence and Dave can answer better than I, but my understanding is that selection and price in BC are not too good. Alberta is the mecca for cheap whisky and wide choice in Canada. Still, you may be able to find some odd bottling that you can't get at home. And the duty-free shops at the major border crossings will likely have some oddball stuff at reasonable prices.