This simple buffer amplifier can be bought from almost everywhere (China). I bough a pair of kits from AliExpress, see link below.
The amp is based on 6N3 (equiv. European ECC85, American 6AQ8 or Russian 6N3P, a rather simple dual triode not really the most linear and well-reputated audiotubes. But it may run on low voltages, making it suitable for low voltage applications.
I bought this amp mostly for the low price and the ability to run it from a low voltage transformer and use it to separate audio sources from power amplifier having a source selector and a volume control.
One of the buffer amps goes with a chip preamp to increase output impedance and the other is a stand-alone housed in a cheap buthandy aluminium chassis from Italy.
There are several different amplifiers based on the 6N3 with between 1 and 4 tubes, but I picked the simplest version with a single tube.
STATUS: Everything almost built to the end. Still have to solder a few wires and screw the box together.