If you qualify under the conditions outlined and are still within the time limit, go ahead and downlaod the application form the Canadian Government site in the first message of this discussion. As Gary indicated, hotel lobbies frequently display pamphlets from refund brokers, that do look surprisingly official...but contain all kinds of restrictions and conditions. I picked up a brochure like that this past week here in town, thinking it was the official Govenment form.
Another option for folks leaving Canada is to get the refund at one of the Duty Free shops on the Canadian side of the border which provide the service free of charge..IOW they don't take a commission and give you the full refund.