Where can I find the Trapper?

At various places in the game, someone called "the Trapper" is mentioned. After I completed some challenges, I got a notification that I unlocked a new item at the Trapper. I got a similar notification when killing and skinning the legendary bear.

I've played a while now, but I haven't seen the Trapper anywhere yet. Where exactly can I find him? Or is the Trapper connected to my progress in the main quest somehow?

Solution 1:

The map above is missing the main trapper in St. Denis. There are 5 in total:

Here's an interactive version of the Trapper map above: http://rdr2map.com?catIds=15

Solution 2:

This should be all Trapper locations: