Lambs salad (corn salad)
Do you mean mache (the French name) or lambs quarters/lambs lettuce? You'll likely find it at Harvest Wagon in Toronto. It's also available in those big plastic salad containers from California -- the kind I like to avoid.
You might also want to read up on edible weeds in Toronto; this is, in fact, a weed that grows all over urban and rural areas here.
Edit: Since I posted this, I've been doing a bit of Googling. I'd been under the impression that mache/lambs lettuce/corn salad is the same thing as lambs quarters, the edible weed. There's a ton of conflicting information on this out there. Anyone know for sure?
It's more commonly known as måche, or sometimes lamb's lettuce. Most major supermarkets will carry it, though it is often called måche mix, and is combined with frisée and radicchio. Specialty grocers such as Harvest Wagon stock just the måche, and you also may be able to find it at the St. Lawrence Market. As for Durham region, I'm not sure, but I would recommend any quality supermarkets, such as Longo's. Enquire with their produce manager. Good luck.
This has been discussed before: