FNC,
Where are you located?
We have birch up here in ND in spots.
Cotton wood is of the birch family I've heard, perhaps you could use its bark??
There's another tree, but I can't remember whic one right now that you could peel bark in sheets from...I'm thinking it might be either choke cherry or june berrry, but I'm not certain...
Good luck and let us know what you find!