Green’s Dictionary of Slang

1. (US drugs) a drug dealer.

1949		N. Algren Man with the Golden Arm 57: Hit me, Fixer. Hit me.
1956		H. Gold Man Who Was Not With It (1965) 102: Maybe he had gone to the fixer.

2. (US drugs) a narcotics user, a person who injects narcotics.

1971

M. Novotny Kings Road 210: With the efficiency of a regular fixer, Helen melted a tablet with distilled water.