I understand there's some official card I need to go through airport security with metal in my leg. How do I apply for that? I don't know where to begin.
These days security personnel are trained for it, and it's a lot more common. So common, in fact, that the Canadian government has an entire page on it:
Essentially, a simple letter from your doctor/surgeon describing the surgery, the implant, and whether or not you're able to go through the scanners for safety reasons will be adequate. Ideally have it on his letterhead, just in case they need to call him (although that's unlikely).
Additional source: I have (several) medical implants too and travelled last year in Canada.