Hi there…
My Fenix HL23 was suddenly out of service and I have received replacement product from Fenix, I can’t be more happy than this.
Now I have “blank host” for headlamp and looking for 1.5 v driver to play around with it. I’ve been searching this forum but found no luck (yet).
So far I can only found this driver from KD but the input voltage is far beyond AA battery.
Any hint(s) is very much appreciated.
Why you dont want to use linear driver with 14500 cell?
Some people prefer AA for convenience and other reasons.
I know I love my eneloops. :D
I've had a similar problem, not being able to find efficient AA/AAA drivers. :(
There was a blf thread where you could order boost driver from reylight. Not sure this offer is still open.
For headlamp, I choose AA (nicd/nimh) over li-ion 14500 because it’s easy to get. I use it mainly for outdoor activity where runtime is expected than the brightness output.
And like Pablo said, I feel more convenient with AA battery attached to my head than lithium-ion.
Last nite I finally use this driver for the mod.
I haven’t figured out how to install the clicky switch but a twisty headlamp seems to be okay at this moment.
I don’t know if you could still find any, but there was a very good EOSLAMP driver available that would handle 1xAA or 2xEneloops, but over-volt and misbehave on two alkys or anything higher - i.e. it could handle 2.4 v, but not 3v.
I have 3 or 4 in mods right now, and CNQG showed nine in stock when they disappeared from the face of the earth. I bought mine from Intl-Outdoors, but they seem to be out. They are low-med-high, always start on low. Oddly enough some of them have a strobe after ‘high’ and some don’t, but the strobe is easily avoided since they always start on low.
They came potted in some black stuff, so I really don’t know what’s on the board or how it’s set up. Two ground wires, one hot wire, and very efficient.
I still have a couple of spares, but you’d have to pry them from my cold dead fingers.
N.B. If there was a driver guru out there who wanted to reverse engineer one of these and perhaps replicate it or even improve upon it, I might be talked into surrendering one. 
I think I remember seeing those drivers couple of years ago.
It was easier to find AA drivers back then.