If the situation down there is anything like the situation here, the best exchange rate is to use your credit card for as much as possible.
Doesn't work for cash requirements, though. For that, it's probably best to go to the bank where you do all your regular banking and get it that way, since you have an account there. For Canadians getting US currency, this is usually the best choice, and I would imagine that the reverse is also true.
Failing that, your US ATM card will work in Canadian ATM's, direct-payment and debit-card stations, and the like, provided that both the card and the machine are compatible with "Interac" (which most are). You'll get dinged with a couple of bucks for the service charge on each transaction, but it works.
There is a last-chance currency exchange at the border crossing points but I don't know how their exchange rate compares. Probably not that great.
If all else fails, most Canadian businesses will accept US currency but don't expect a good exchange rate.