I have an old incandescent Surefire E2D and I'm curious if I can use the body and cap to make a different light just by changing the head. Which heads can I use and where do I get them? Does anyone have one on the ones from eBay?

Malkoff VME head and M61 drop-in fits E series.
So does his Scout head replacement unit.
A single cell rechargeable in 17670 will run a hour or so and then drop down slowly once it reaches 3.4v which drops it out of regulation.
You can even use cells removed from bad laptop battery if they are 17670 but you may have to get the body bored to accept them.  
16650s will fit without modification.
It can also run on 2 CR123 cells or two rechargeable 16340s if you like.
You may want to read up on Liion safety before you do this.  

The Scout replacement head is a little smaller than the VME and con be ordered in single or three mode versions.

I have built so many new Surefire lights from older parts. I usually use the mh90 body from the m951/61/52/62 but you can definitely use a ed2 body and tailcap. they even sell adapters so you can use 1" heads like the ones on m952 on a scout/ed2 .75" body. You can buy conversion heads or bulbs with a bezel. You can basically build whatever you want out of your ed2 body. EBay is probably the easiest place to find every part and then some but be careful of some of the cheap stuff.

Malkoff is worth the $

Another vote for malkoff. Tough, well made, bright and not crazy expensive.