So, I've done some more digging, thanks to Victor:
http://www.lateromanbronzecoins.com/magnus-maximus-more-sources/As far as sources are concerned, I've seen the following:
Early sources (fourth and early fifth century):
Ammianus Marcellinus – Rerum Gestarum Libri Qui Supersunt XXXI.4.9
Orosius – Historium adversum paganos VII.34
Pacatus – Panegyricus Latini Pacati Deprani Dictus Theodosio
Prosper (Tiro) of Aquitaine Chronicon
Socrates Scholasticus – Historia Ecclesiastica V.8; V.11
Sozomen – Historia Ecclesiastica VII.13
Sulpicius Severus – Dialogi II.6
Sulpicius Severus Dialogi III.11
Sulpicius Severus Historia Sacra II.49-51
Sulpicius Severus Vita Sancti Martini XX
Later sources
Historia Nova, Book IV, Zosimus c. 500
De Excidio et Conquestu Britanniae, Gildas c. 540
The Pillar of Eliseg, 9th century
Historia ecclesiastica gentis Anglorum, Bede, c. 731
Historia Brittonum, Nennius, c.828
Breuddwyd Macsen, The dream of Macsen Wledig, 11th century
Historia regum Britanniae, Geoffroy de Monmouth, c. 1136
The five royal tribes of Wales and The fifteen tribes of Gwynedd, 15th century