On the off chance anyone's still looking for recs here: https://dark-solace.org/elysian/viewstory.php?sid=6999
A New Sheriff in Town by kats_meow
Author's summary: Thanks to Halloween night, a frightened town, and enchanted costumes courtesy of one Ethan Rayne, Slayer and Slayer of Slayers were magically thrown together. Now, no matter how much either of them claims to hate it, their partnership must continue if they hope to stop a full-fledged reign of Chaos on the Hellmouth. Look out — Buffy Summers is gonna lay down the law.
Buffy decides to exchange the fancy dress for a sheriff's costume, Spike finds his way into Ethan's and winds up dressed as her deputy, and shenanigans ensue.