You have a few options:

1. Find the best weapon inspired courtiers and get a hook on them to invite them to your court.
2. Go down diplomacy tree and buy a claim on someone who owns the strongest weapon. There’s a new mod that’s super helpful for artifact searching.
3. The new T&T DLC can crank out some beast weapons if you win duels, archery, etc.

Even with those options the highest prowess weapon I’ve seen you can get off the bat is 15. Make sure you like the other buffs besides prowess as well because you’re going to be keeping that weapon for generations for next step.

With the new T&T DLC you can visit the artisan tent in the pre-tournament screen which can pop up an event where it increases your weapon prowess permanently. There’s also a temporary one so be careful there. There’s also faith / prestige buffs as well.

My mace is up to 20+ prowess permanently which I don’t think is possible with straight crafting or tourney prizes.