Does anyone have a source reference for Advayavajra in tantric buddhism?
If by Advayavajra they mean Maitripa then it must come from Maitripa's Amanasikara.

Specifically, in Mahasukhaprakasa, text #16 in above translation, so pages 181-187.

In addition, for an English translation of a small but relevant part of Sadhanamala see "Two Tantric Meditations: Visualizing the Deity" by Luis O. Gómez. in Buddhism in Practice pages 236-245.