I bought the Canon-specific housing. A friend of mine had one, and I've borrowed his before to go scuba diving. He's never had a problem, and I trust his judgement. Got it brand new from B&H. I wouldn't mess around with used housings, just to be safe. Here's a link:
http://www.bhphotovideo.com/c/product/526142-REG/Canon_2319B001.html
looks like the price dropped $10, since I bought it, too.
As an alternative, Ikelite does also have a housing, but it seems that the price difference is for the rated depth (Canon=130', Ikelite=200'). I don't ever intend to take a camera past about 100', so the additional cost didn't seem worth it, in my opinion.
From what I learned from my buddy mentioned above is that the key to these housings keeping their seal is good maintenance and upkeep. Always make sure to thoroughly clean your housing before opening it. Not one spec of sand should be on or near the housing when it is opened. Also, thoroughly rinsing the saltwater, too. Salt crystals can cause leaks in the seal. Lastly, making sure to always keep the rubber o-ring (gasket) slightly greased with the provided grease (not too little, not too much). Again, I'm basically just repeating what my buddy taught me, so I make no guarantees.
