With the help of DuckDuckGo, I was able to trace a long corporate history, starting with the original Eitel-McCullough company (1934), which became Eimac. Then, a merger with Varian Associates, ending up as a part of Communications and Power Industries (CPI).
The good news is that, within CPI, the Eimac name still exists, and they still market power transmitting tubes. Alas, the classic glass envelope / internal anode tubes (as shown above), including the 3-500Z, are no longer offered by Eimac. (Some are still widely available as imports and marketed by firms such as RF Parts.)
Eimac is found in high-end markets for commercial, industrial, and military transmitters. Glass has given way to ceramic for insulating seals, while external directly cooled anodes are the choice for efficiency and performance. "Low end" Eimac tubes (triodes and tetrodes that are effective for our 1.5 kW power levels) may still be produced in low volumes, but they will be very expensive compared with glass envelope imports for our ICAS operations.
Over recent years, the old vacuum tube technology has been replaced by solid state designs, especially for commercial service. So there is little demand for the kW size tubes. Still, it's much cheaper to keep an SB-220 going with new imported tubes if needed, than to upgrade to the latest transistor amps.