The usual sources for these hulks is pretty well know to those who have an interest in these undercarriage pieces. Many still reside in logging company 'bone yards' but the proximity to tidewater and China have had many of these old vehicle dumps scrapped out. Close to where I live, there is a business who receives a lot of salvage machines and over the years a few yarders and spars. Madill in Nanaimo on Vancouver Island used to be the place and as a youngster a 100 yrs ago, I recall seeing armour in their compound waiting for conversion. Sadly, that business is gone and the jobs with it are now history. It would be interesting to see if pictures of their operations exist and might show the 'before' and 'after' of these machines.
Not to send anyone down another rabbit hole, but here is a link to a discussion on Madill and some pictures of their stuff.
http://www.heavyequipmentforums.com/...-in-Nanaimo-BC