Where can I buy an iPhone 3GS in Kitchener Waterloo?

I know that apple no longer produces 3GS's but I'm looking for stores that might sell older electronics where I can find one.

You might try Factory Direct on Victoria North:
Link - http://www.factorydirect.ca/
Or keep on going down Victorian North to XS Cargo across from Bingeman's:
Link - http://www.shopxscargo.com/

No guarantee they'll have what you're looking for.
But I do know they sell older electronics so they might have something you'd like.

Put an ad on craigs list or kijji, there are lots of people with old phones kicking around