It'll probably be cheaper to buy a tank, but first you got to find out what the local suppliers' attitudes are towards filling/refilling it. Don't know about Canada, but in the USA there doesn't seem to be any one set of rules - some don't ask any questions at all, other will take a DAN oxy administrator card as credentials, and others just won't sell it period.
You can put together a medical O2 set pretty cheaply on eBay.
Alternately, you can get a scuba tank and have it filled with O2 at a shop catering to techies, and use an O2-cleaned scuba regulator to administer the O2. This will usually give you better duration (larger tank and demand rather than constant flow regulator) but is less effective on an unconcious person.