I've ordered from Summit racing, and bought some tires in the US before the exchange went for a crap, (it is now cheaper to get BFG's here in Canada.) The general rule of thumb is that if the part is made in the US, there will be no duty on the item, thanks to the NAFTA, just the GST and PST (payable on the exchanged canadian $ amount of your purchase on that given day, not on the US dollar amount). If you get your purchase shipped to a close US city, and will import it yourself, be sure to get whoever you are buying the parts from to include a copy of the waybill, with the prices you paid for it. This will save you a ton of grief, proving to some overzealous customs officer what you actually paid. If you don't import it yourself, count on having to pay a fee for the customs broker, I can't say how much offhand. If you need more clarification, call your nearest customs office, they should be able to help you, they're supposed to be public servants. They might even be able to point you in the direction of a couple of brokers.
Hope this helps you out a little.
Have you also tried National 4WD online - www.national4wd.com
they are located in Burlington, ON - a little far from you but still in Canada.
How about www.interioroffroad.com
- I haven't tried to buy from them, but they are a lot closer to you, and seem to have a good online catalog, though I don't recall seeing any exhaust systems.