If you've got an old stereo .... I'd just roll yer own.
Build a headphone patch panel that consists of 1/4" stereo jacks, as many as you need to get a headphone feed to everybody.
Wire each jack through a separate 2.2K 1/2W resistor in series with the positive leads for L and R channels.
With the 2.2K common resistor value you should be able to run headphones with impedances from about 4 ohms on up to a couple hundred ohms without any problems.
Good idea to use impedance matched cans, though if you don't install separate volume control L-pads as described below, the guy with the 150 ohm cans will have lower volume while the guy with the 8 ohm cans will get blasted.
Wire the paralleled jacks and resistors to the SPKR terminals of the unit.
You can easily and safely run at least 20 sets of headphones maximum this way, and most don't need that many.
If you need separate volume control for each headset simply wire in L pads, 8 to 16 ohm wire-wound dual pots designed for stereo speaker fading, which you wire in after the resistors to each headphone jack.
L-Pad goes between each resistor and it's corresponding output jack, pin 1 to hot, pin 3 to Common and pin 2 to the jack output.
Other than that, you could check out some of
these.
