I built this circuit so that I could use a surplus headset that is, in essence, a military headset. M-87/A microphone element, 19 ohm earphones, UG-174/U connector, the whole nine yards. I power mine from a 9 volt alkaline battery and can easily get 40+ hours out of it, as long as I remember to turn it off when I'm finished flying!
The circuit is fairly straightforward and uses no exotic parts, except for the UG-174/A jack connector. Indeed, all the parts (except the UG-174/A jack) can be found at Radio Shack. The UG-174/A jack I used came from an adapter cable that came with the headset. The manufacturer, Nexus, has a web site and apparently will sell them to anyone but I have not tried them yet. Here's a link to Nexus. Looking through the Aircraft Spruce & Specialty catalog revealed no UG-174/A connectors.
The only adjustment required is the A/C Mic Out pot, which sets the output level of the microphone amplifier. The best way to do it is with the headset & adapter connected to an aircraft radio, and checking the radio's transmit modulation with a modulation monitor. Adjust the A/C Mic Out pot for best modulation without excessive clipping. If you don't have a modulation monitor handy, set the pot at midrange and try to get reports from other pilots on how it sounds. The Volume Cntl pot is set to suit your needs. I usually end up with it at midrange.
After taking a long look at the schematic, I can see where the circuit could (and should) be simpler. For instance, the 600 to 600 ohm transformer on the mic output circuit isn't really needed. It is there because I wasn't completely sure how to interface to the aircraft radio when I designed the unit. By the same token, there's no real reason for two op-amp sections in the mic amplifier. One op-amp section would be sufficient. The receive amplifier is about as simple as it can get.