They only have to be serviceable, which is manageable just like they were made in from antiquity to the modern era, wood. It might take some practice, but a hand-carved shaft that has the tip dipped into molten lead or pewter could make a rather effective bolt. You could use more modern materials as well; various types of piping, scavenged hardware like nails and dowels, etc. They may be less accurate and harder on the crossbow, but they don’t need the longest range in this topic and the wear and tear on the crossbow may be justified if you know how to repair it and maintain it to ward off failure. Obviously, there would be trial and error, but it wouldn’t take too long to become a competent Fletcher.