The best place to get cash exchanged is probably the VBCE...
http://www.vbce.ca/
It should always be at last a couple points cheaper than using your credit card.
I know that both my PC Mastercard and MBNA Mastercard both explicitly state that they charge an extra 1.5% fee on currency conversions.
However... if you're not dealing with large denominations, it's not that big of a deal... maybe the convenience of using plastic is worth the extra fee.