In London I bought them at the Loblaws by Masonville (or the one on Wonderland if I was out that way). Here in North Bay they have them at Sobeys and Independent Grocer too. Generally if it's a big grocery store and they have one of those trendy 'natural foods' sections they'll have them. Most health food stores seem to have them too - I popped randomly into one in Toronto in Kensington a while back just to see if they had them and they did!
They *are* pretty expensive though... certainly not 99 cents! More like $1.50 a piece... I probably won't keep buying them when I go back to London in the fall but for now my mom is happy to put them on her grocery bill for me, lol