A stroller shop that specializes in strollers is your best bet for a good purchase.

Big Box neighborhood stores sell them but tend to have a poor selection of models. 
An online store sells only strollers, have an extensive selection, far better pricing, and live customer service with very knowlegeable staff. 

Most online stores have clearly established return and warranty policies, and many offer free shipping with the continental USA.  Many also sell to Canada.

Here is a list of the more established, and reliable online stores:

Store	Links & Comments	Ship to Canada?
	Strollers.com carries a wide range of jogging strollers.  They stock many models in the discount brand categories (i.e. Schwinn & Instep) as well as the high end brands.
	Check Out their wide selection of double strollers
	Shop at HealthCheck Systems for Everything You Need for Your Baby. Get an Extra 5% off any Baby Product with Code BABYFIVE.
	Check Out the Phil &Teds Dash Buggy double stroller here!
	Buy brand name doubles for shipping to Canada
	Find discount brands like the Schwinn Hope Jogging Stroller at Amazon: a name everyone knows and trusts!
	JoggingStroller.com also specializes in jogging strollers and accessories.  Their staff is very knowlegeable, and they stand behind their products with a no-hassle return policy.You'll find all the popular brands in the trendy colors like the Chariot Cougar
	BabyAge is a well known baby portal with an extensive selection of double strollers and other baby gear