Where can I find a reference for Greek vocabulary in Katharevousa?

Any dictionary of Greek before 1970 is going to be biased towards Katharevousa, and that includes any Greek dictionary you find online (legally). That includes, for example, the 1868 Contopoulos English–Greek dictionary, Νέον λεξικόν ελληνόαγγλικόν. It includes the 15 volume Dimitrakos monsterpiece (not linked, since bootlegged). It also includes any number of Greek–Greek or Greek–French dictionaries, such as Dehèque, Hépitès, Koumanoudes, and Skarlatos Byzantios. You’ll find all of these on Google Books or archive.org

The lexicographers at Trapp’s Lexikon der Byzantinischen Gräzität in Vienna use Stamatakos’ three-volume Dictionary of Modern Greek (Λεξικόν της νέας ελληνικής γλώσσης, καθαρευούσης και δημοτικής και εκ της νέας ελληνικής εις την αρχαίαν / Ιωάννου Σταματάκου, 1952) as their reference for Katharevousa.