I use a book called Canada Post
Offices 1755/1895 by Frank W. Campbell. This book lists all Post Offices that were opened in the period in Canada. It's not an expensive book. About $15 and you can find this easily on Amazon
, etc. If you want to do this online, you can join the Postal History Society of Canada (PHSC). They have an online e-Members fee of $15 per year which gives access to an online datanase. I haven't seen the database but heard that it is good.