They are still being made but are mostly surface mount today. I saw some for several dollars each. The best price I've seen is around 3 for a dollar in larger quantities. The ones above have no pin installed. I was thinking about making something like them. Still considering the construction details. I'm storing this info until I get some shop time for the actual project. You might ask what are the good for?
The three terminal capacitor IS a feed thru capacitor. My low frequency application will be using them for isolation between stages. For example when feeding a DBM with a RF signal and a LO signal the stages must be isolated. The leads act as antennas radiating and receiving signals. To prevent the cross talk the stages will be separated by metal shields. The signal and supply leads will feed thru the shield using feed thru capacitors. I have a few on hand but intend to make some for this project. Next step will be to get the shielding and breadboard.
HINT: In microwave the vias have capacitance and act as feed thru capacitors.