The key parts are:
Small speaker, such as Radio Shack part number 273-092 for $3.69
Condenser mic, such as Radio Shack part number 270-090 for $2.89
4-conductor 3.5mm stereo plug. I haven't found one I like, but part number 171-6435-EX ordered from Mouser (http://www.mouser.com) for $2.79 will work.
If you're the sort of person to build this, then you're also the sort of person to have all the remaining parts lying around. If not, the resistors, switch, connectors, and wire can all be bought from Radio Shack for a few dollars apiece. I found that the cheap 1/8" stereo phono jack sold by Radio Shack (274-246 for $2.69) is a bad idea since the contacts oxidize after a year or two and the audio gets intermittent. Instead, I'm now using a "gold series" jack cut from the end of their 6-foot stereo cable (42-2482) which is expensive at $15.79 but has worked great for years. One could use the rest of the cable for the harness side; I happen to be using half of a 6-foot stereo patch cord I bought from Target for about $4, and it has been surprisingly reliable. I don't know the source of my PTT finger switch; some people use Mouser's part number 101-TS6424T2601-EV for $0.76. The wire I'm using that runs down my arm to the PTT switch is 20-gauge speaker wire (Radio Shack 278-1388). It's OK, but fails from flexing every couple hundred flight hours, so maybe someone will find a better option.
Brenor Brophy provides his own design here: http://www.brenorbrophy.com/HG/head_set_ptt.htm, complete with excellent detailed instructions. His design has more connectors, which are more parts to come unplugged or fail, but they do improve convenience.