The City of Calgary has a web site on Poplar Trees... they claim the cracked foundation thing is a myth:
http://www.calgary.ca/portal/server....ar%20Roots.htm
As for where to buy, some of the nurseries in the city have gotten stupidly expensive in the past two years. I would say the Heritage Costco, but it's almost too late now, there tree stock is probably sold out. They had Swedish Aspens for $39.95, but unfortunately the second batch of the ones they were selling had no root system and I have already had one die this year.
I would almost steer clear of Swedish Aspens at this point. They have gotten far too popular and could be susceptible to a disease outbreak. Sutherland Caraganas are a good alternative.
Don't bother waiting until late in the season hoping for a sale. You are better off buying early and being able to chose the healthiest specimens.