http://www.civilization.ca/cmc/exhibitions/cpm/catalog/cat2104e.shtml#1222124

Beginning in 1908, mail-order homes were advertised in the catalogues on both sides of the US-Canada border. They were wildly successful on the prairies, where sod houses were replaced with houses that came on the train, all materials and plans, to be erected by a carpenter on site. One company, the Canadian Alladin Company, sent the entire house, packed flat, all the pieces numbered, for easy assembly. The T. EAton company began selling houses from its 1910 catalogue, but only in the west. The article at the link above, has more information.

Growing up, I lived in houses built by the Ontario Hydro for its employees. I assumed they were built on site, according to plans form the company. However, one source indicated that the Ontario Hydro was a purchaser of the mail-order houses.