In the PHB, the Elemental Weapon spell has a stronger effect when cast using a spell slot of 5th level or higher, with a 7th-level slot described as providing the strongest effect. However, in the PHB the only class that has the spell on its spell list is the Paladin, which only gets up to 5th-level spell slots according to its advancement table.
What methods (if any) are available for a PC to be able to cast Elemental Weapon using a spell slot higher than 5th level?