Macarons in London, Ontario?
You might want to call a couple of smaller bakeres like The Village Bakery in Wortley Rd Village http://www.villageharvestbakery.com/, not sure if they carry them but I have family in London that used to speak highly of this little place - even if they don't have them, I'm sure they might suggest a place that does.
Petit Paris has opened at the Covent Garden Market, close to Doris Family Produce, and across from International bakery. They sell macarons, other French sweets and pastries, savouries and other take-out foods. It's owned by Nicole Arroyas, who also runs Auberge du Petit Prince.
Auberge Du Petit Prince
460 King St, London, ON N6B2Y9, CA
Covent Garden Market
130 King Street, London, ON N6B, CA