I'm looking to buy a folding bike for my commute to work and want to spend $350-400. The bikes at Vela.ca look very good but I want to make sure I get the best deal. Shipping costs are also a concern. Vela has a flat rate of $35 which is very low.
Downtube is well-thought-of, and they ship internationally. They also accept returns, although you'd be out the money for shipping.
Dahon has several bikes under $400US/$412CAD - the Boardwalk and Eco (and the 7-speed Speed is you can stretch your budget a little). Dahon is likely the best choice for a budget folder, and they have dealers in Canada.
I'd suggest starting with your local bike shop. They're the ones that will do some of the servicing (I'm guessing) and having a good relationship with them is always important. If they can't even come close to pricing or won't get a folding bike for you then your local Craigslist might yield some results.