just bought one from telus this weekend. Had 13 months left on my old contract. Got it for $399. Basically cancelled my old plan with no penalty and signed up for a new one supported by the Iphone.
Craigslist is your friend.
Or try eBay. I'm sure there's tons of iPhone 3Gs being dumped now that the GS is the standard.
But really what is the point of an iPhone without the data?