In Ontario, I know of no policies available to non-car owners for the purpose of renting. I've looked into it for myself with no luck. It's true this is not the case everywhere. It's best to look it up before you rent because the sales tactics at the counter can be intense.

Car rental companies have mandatory coverage for anyone who rents their vehicles. Everything they offer you goes to their bottom line. If you don't buy their collision damage waiver (it's not insurance) they will charge you for damage and loss of use of a vehicle no matter who is at fault. However, they will also file a claim through insurance. They get paid either way.

You cannot purchase 3rd party liability coverage from a a car rental agency in Ontario. The agency has it in their corporate policy. You can buy personal accident and collision damage coverage, but because any decent credit cards will have their own, why bother?