Let me see if I've understood you. You are the DM. Your players have appropriate PC wealth, but didn't spend it on items that would have made them more effective. So now you want to give one of the players, who plays a Paladin, a special magical weapon.
Do I have that right?
Seems to me that if the players have the appropriate wealth, they get to choose how to (mis)spend it, and they are the ones who have to live with the consequences.
In any event, try a cold iron longsword, +1, holy, with an appropriate Bane property.