I found a used one for mine a few years ago. It was hard to locate a new one that wasn’t selling for twice the MSRP. The housing from Canon for the A620 is the WP-DC90. I had heard they were planning to rerun this housing if they got enough response for them. But not sure if they ever did it. Good luck on your search.
Here’s a used one on Amazon.
http://www.amazon.com/Canon-WP-DC90-Waterproof-Case-PowerShot/dp/B000B65GCA