Aquasport in Marda Loop are excellent. I highly recommend them.
I assume by saying snorkel you mean snorkel and mask, because a snorkel on it's own isn't much good...unless you already have the mask. Get one with a decent mouthpiece because you'll be spending a bunch of time in the water and biting into it. Just don't buy one from Walmart because the plastic mouthpiece will likely hurt as it's harder plastic. Make sure you get good silicone.
For the mask, hold it up to your eyes, without putting the strap on and then breath in through your nose. If the mask sucks into your face, and stays on your face you're getting a good seal. A poor seal will wreck your snorkelling because you'll always have to clear your mask.
MEC also used to carry a US Diver set which looked like it would do the job. They still might (edit:
they do ). The ones at Coast might also be ok.
Make sure you wear a dry fit shirt because if you're like me you're out puddling around in the water for an hour or more at a time and sunscreen just doesn't cut it.
Enjoy your foray into the magical underwater world...maybe if you snorkel without a hitch you'll want to scuba. Aquasport is again the place...unless you're down south then Adventures in Scuba are fine.