Okay just finished farming for these two heads and seeing as there is little info on custom heads all around i felt i should share...
The first head piece for maya is "till death do us part" it looks like a bridal veil. drops from the sub quest boss "henry", from the quest line "best mothers day ever". Once you complete this quest chain henry becomes easy to farm just warp to the highlands overlook and run off the cliff jumping down to the den henry will spawn. Take him down, grab the loot then if you didnt get the head piece, quit and then click continue. You'll start back at overlook, then drop down to the den and fight again. Rinse and repeat. I probably killed henry roughly 120-140 times before this dropped.
The second head pieces i found was for the gunzerker called "the bandit" its a cap with goggles and the second was called "shrapnel something..." (cant remember the name im on my phone i'll update later) its for maya and its a helmet. These two head pieces i got from the boss bn-k3r. From the main quest "where angels tread". You cant warp to the bunker but once your there you can kill him farm him, if the head didnt drop you can quit and then continue and you'll be at the entrance of the bunker once again. Rinse and repeat. *note* check under the platform where the loot drops some falls under the floor. I killed bunker 3 times for the zerker head and another 200+ for maya's
I also got enough eridium from doing this to buy all sdu upgrades and have 99 eridium on the side by selling weapons from these two bosses i earned roughly 1,700,00 bucks and 4 challenge skins ( i unlocked them while farming) i did all this at level 31 no issues, fast runs. This was done solo (may be faster drops with friends) full healing spec. Hope this helps people! Good luck!