Hi Bagger,
Welcome to Bannerlord Online!
Bolts and Crossbows can drop as random loot from any units that actually use them. For PvE, that generally means having to do the headhunter mission for the Commanders located in cities. Vlandian troops have the best odds, but they're also more dangerous to engage without a proper army of your own. (This mission can be done 5 times every 12 hours)
Aside from that, bolts can also randomly be found in the hidden chest as part of the Mercahtn quest given by the thief located in taverns. (This mission can be done every 5 minutes, if the thief offers it)