Hi,
There was a lot of Praxian based supplements from the Tentacles crew under the name of Ye Booke of Tentacles, which had multiple scenarios and backgrounds written by Ian Thompson. There were six substantial books produced.
As for supplements from MRQ, I would add the Player's Guide to Glorantha for the sections on Praxians to the lists already provided.
Borderlands, Pavis and the Big Rubble and the Gods of Glorantha books by Moon Design can still be found. I have used all of these with MRQ mechanics, as well as Griffin Mountain. I didn't actually change the stats much, I just reduced the number of Battle Magic/Rune Spells and decided if an NPC was a Spirit Worshippper (most Praxians and Balazarings), a Theist (Most Orlanthi and many Lunars) and the occasional Sorcerer (I had a couple in my Pavis Game who were in the service of the Lunars, amongst the Trolls or amongst the Old Pavis worshippers using exotic magic.
I do know that the new Pavis book connects the Second Age Material and Third Age material together much better, so Dragon Mysticism and the legacy of the God Learner's magic in the former Robcradle can be found, allowing the player to draw upon the Magic of Glorantha supplement.
Finally to save the 3rd Age GM a bit of time, if you are interested I have converted Star Bear, Yelmalio, Etyries, Cacodeamon and the Seven Mothers cults to MRQ to match the changes that occur in the Third Age. PM if you want a copy.
Simon