I don't know if this company will sell/ship to Canada, but you might try radiator.com. I ordered one from them for my '92 SSE supercharged last week, and it was roughly $175 US, delivered overnight, with a lifetime warranty. They were the very polite and knowledgeable, too. This radiator is the one with the aluminum main core plus a plastic tank on each side, the coolant level sensor port on the back of the plastic passenger side tank, 2 fittings on the driver'* side for the engine oil, and 2 fittings on the passenger side for the transmission fluid.
By the way, the factory shop manual says that the plastic tanks can be purchased and replaced separately, but I don't know where you'd be able to buy one, or if it'* help you in your case.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles