I've tried pretty much all of them in the GTA and have found only a couple with decent food with consistent quality and good value. It depends what you are looking for. If you want fast food, check out Mucho Burrito. Its one of those fast food type joints appearing all over the city (usually near movie theatres like colossus), but the food is quite decent! For one thing, their hot sauce is actually HOT and the pulled pork has some real flavour :)
If you want Tex-mex style food, then check out El Sombrero (just south of 401 and hurontario...same plaza as My Apartment). Food is really good - especially the burritos. But, keep in mind, its more tex than mex :) Still, much better than your average mexican place.
Finally, if you want really nice food with more of an authentic touch, then for me, there is no better place (out of the ones that I have tried so far) than Little Mexico in Vaughan. Its just east of the big auto mall there with 7 dealerships. I think the street is called "woodstream". If you go there, try their Tortilla soup (yummy!), item #17 (I think its pork ribs in tomatillo sauce), Chicken mole (really nice...NOT sweet, but very flavourful!), chicken enchiladas and tacos. If you like it hot, ask the server for the special hot sauce on the side. Stay away from the margaritas, though. Not worth it. Get a Tecate, instead :)
Let me know what you think after you try them :)
I had trouble finding them, also. If you travel to the US at all, you can pick them up at all the grocery stores. Heck, even the Dollar Tree in Niagara Falls (USA) has them for a buck! Over hear in the GTA, I've had more difficulty finding them in the normal grocery stores.
I have gotten them in the past from some of the restaurants I frequent - like Little Mexico (24 Woodstream Blvd Vaughan) or El Sombrero (Britannia and 401 - same plaza as My Apartment). So, if you are really desperate, ask the owners in either of those two :)