Duck fat in London, ON?
Go to Chris Country Cuts at the Covent Garden Market, buy a duck, roast it, and retain the drippings. That's where I've found duck fat in London. It's in the drippings. ;-)
Chris Country Cuts
Convent Garden Market
If you just need a small amount of duck fat, Remark Market has duck breasts in their freezer section, and each duck breast would give off at least a couple tbsp of drippings once cooked. I guess you could thaw the breasts, trim off the fat, and save the duck breasts for another recipe.