I think your best bet is to find a local brewery and see if they'll sell you a keg they're going to decommission. Pretty much any other method will be in a grey area as far as the laws are concerned and will start quite a debate.
Been looking for 3 - 15.5 gallon kegs for a brewsystem for a couple of months with no luck. tried Kijiji and CL. phoned several breweries in Edmonton with no luck. If someones got another suggestion I'd love to hear some suggestions.
Local establishment owners often will let you "borrow" kegs if you pay the $30 - $60 deposit, if the morality of that is too much for you to handle, when new owners take over an establishment form time to time they've been know to change distributors, often the old distributor wont be bothered to pickup the old kegs cause the deposit been paid, by the previous owner. Abandon kegs are often free or cheap.