[AzraelDarkangel]

The best I've seen are the Turbulent-X drivers from Turbulent Labs. Most of the other headphone elements I've found for sale are for lower end headphones. Can you buy high end Beyer, AKG, Audio-Technica driver elements?

 

[PETEREK]

The absolute best drivers/headphones you can get for DIY that are fairly cheap are the Fostex T50RP. Hands down. There is so much you can do with them, and you can curve the sound in a ton of different ways. If there's a specific sound you're going for, chances are someone already did it and can tell you how to go about achieving it. 
 
Here's my pair, all DIY done by myself.
 

 

 

 

 

[ProtegeManiac]

The best I've seen are the Turbulent-X drivers from Turbulent Labs. Most of the other headphone elements I've found for sale are for lower end headphones. Can you buy high end Beyer, AKG, Audio-Technica driver elements?

 
You can buy Sennheiser HD600/650 drivers, however I have no idea how you can DIY them without a lot more customization. For example, it might be interesting to mount them in a custom spec chamber (dimensions like depth) and also control their distance from your ears depending on how you design the other side of the chamber where the earpad goes, as well as with the earpads themselves (such as using angled AKG K7xx earpads); however, the problem then is that the drivers come installed on the oblong baffle where the HD6xx grill mounts on one side, the earpad on the reverse side,  then the headband forks attach to the sides. If you can construct a chamber and earpad mount similar to a customized Grado like this, I'd be interested to hear how they turn out.