basically the subject...

There is a small scene in Heartbeats by nerfherder (on this site) where they get high.

They get high in the AH fic Aloof Rocker Kryptonite.

Aloof Rocker Kryptonite by JustTiff

Summary: Spike's a rock star, but Buffy hasn't heard of him, initially. Buffy's a 21 year old college girl on summer break, just hanging out at her step-brother's bar. Guess what happens next.

The first story in The Baby Series by Mrs. Muir, Baby Did A Bad Thing, has a scene where they get high in graveyard.
Summary: Spike is called away by Drusilla before he can meet Buffy at the Bronze in Tabula Rasa. He doesn't return to Sunnydale until after the fiasco of Xander and Anya's wedding. Things aren't the same as when he left town...especially Buffy.