Linen is an excellent natural material for bowstrings and fairly easy to obtain:
- Irish shoemaker's lockstich thread: recommended by Baker (2000c: 114-115)
- Bookbinder's linen thread: recommended by Payne-Gallway (1990: 110) and this article's author. This type of thread is available from here.
The sailmaker's sewing twine Payne-Gallway (1990: 110) suggests for heavier crossbows is probably not available in these days, and it's unclear what material it was.