No info about availability.
Ring modulator using vacuum tube in audio circuit.
Erica Fusion Ring modulator module uses pentode tube and it multiplies two signals to create unique waveforms rich in partials, bell like sounds, as well as tube distortion effects.
http://www.ericasynths.lv/en/shop/eurorack_modules/4550-fusion_ring_modulator.html
These merchants probably sell this module. Huh?