there is a place on the danforth west of pape on the south side called the Greek House. They have two different varieties of halloumi. I don't know what the difference is though. I do know halloumi is great when you rub olive oil into it and throw it on the grill. Enjoy!
I'm not sure about quality, but the IGA at Danforth and Pape has it. I remember noticing it a little while ago, but I neglected to pick any up. I'm sure lots of other stores around that area carry it, too.
I have found it a couple of times at different spots at the St. Lawrence Market. Unfortunately the names of the spots have escaped me - I beleive I found it the last time at Scheffler's Deli in their dairy section.