Bumbling Along is (so far) the only Canadian link I have found.
I’m hoping to find more but.. there are two main sites which have world wide links and resources.
Letterboxing is a outdoor recreational activity which combines collecting, puzzle-solving, and treasure hunting.
Briefly, letterboxing enthusiasts hide and hunt weatherproof containers in remote or scenic places. Each container holds a guestbook, a rubber stamp and stamp pad. The planter of the letterbox distributes clues to its location; solution of these clues may require a combination of skills such as mapreading, orienteering, and puzzle-solving.
Letterboxers carry their own stamp book and personal stamp when hunting for hidden boxes. Upon finding a letterbox, they will impress their own book using the found stamp, and leave their own stamping or personalization in the letterbox guestbook.
Letterboxing has its roots in Dartmoor, England, in 1854.
Description of letterboxing from Curlie, the category for letterboxing links.