There is the Spirit Marsh Dungeon, which is a purchasable elixir for platinum as well as craftable. The Spirit Marsh is a zone with enemies locked at level 67 give or take party size, so at 71 they will die much faster and thus result in faster leveling of the spirit weapons.
Direct purchase to level up a spirit weapon isn't off the table, but currently not something we're looking at doing with the expansion so fresh.
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