6678 / 6U8A
This is a suitable replacement for any 6678 / 6U8A tube type. Various brands.
Since all worldwide 7199 tube production has ceased and is not expected to return, the availability of this tube has begun to dwindle and subsequently the cost is quickly becoming prohibitive.
Thankfully, there is a solution by implementing a simple wiring change and substituting the plentiful (for now) and otherwise identical sounding NOS 6U8A tube in place of the 7199.
See how to here.
If you do not want to re-solder your socket, get the plug and play adapter here.