Birchbark is probably the best, but getting it at the right time is the challenge. The harvest is apparently limited by the states. However, I seem to remember that our guy, Swampman, moved up into that part of the country - maybe we can jar him into being a supply source! May also try contacting the various bands of the Ojibway nation to find a supplier. They are spread out over most of the birch range.
Hawkdancer