Presuming these look like later bills, I could think up of some simple conversion ideas:
- metal spearmen standing, with the weapons replaced by separately bought metal bills (easiest done with open handed figures, but you could also drill through hands)
- plastic 13th century infantry from Fireforge, with bill-bearing arms from Perry WotR plastic infantry (presuming these look good when mixed together)
In case the bills would look different from 15th century bills, then the issue could get a bit more tricky.