Lutejones - Cottonwood is the name of more than one tree in North America.
Sometimes called Black Poplar, or Balsam Poplar (there are other names too). Sometimes these trees get quite large and develop very thick bark, at times 3 or 4 inches or more thick.
When the trees die, the bark is pretty easy to remove and often is quite good to carve. It is easier to carve than most wood, yet takes good detail and has a pleasing grain.
It take a fair bit of hunting sometimes to find a good bark tree, but once a good one is found, it is like a little gold mine. I have one that I know of that has produced quite a bit of bark for me to sell and to use in my carving courses.
I cannot speak for everywhere, but I personally know that good bark can be found in British Columbia, Yukon, Alaska, Alberta, Washington, Idaho, Montana and I am quite sure, most of the northern states.
It cannot be shipped across the Canada/US border.
I live in BC.
Conventional wisdom (?!) has it that the best bark comes from the Northern part of BC, and that may be true, but I have found some very good stuff here in the Okanagan Valley of BC.