Or you can build a Christmas Tree like me
These are Novosibirsk 6S2S, the Soviet version of the 6J5GT. As
@triod750 mentioned, you can also use the 6C5 family of tubes. I have a pair of almost 80-year-old VT-65 Ken-Rad's that, in a blind test, I'm pretty sure I wouldn't distinguish from the 6HM5.
There are literally hundreds of triodes that can be used in Little Dot. For some of them, there are off-the-shelf adapters; for others, you'll need to build a specific one.
Here's the list of families I found compatible for drivers (6,5V, <= 450mA):
2C22, 37, 3A/167M, 437A, 5842, 6AC5G, 6AC5GT, 6AE5GT, 6AF4, 6AM4, 6AQ4, 6C2, 6C5, 6C8P, 6CW4, 6DS4, 6ER5, 6ES5, 6F12, 6F5, 6FQ5, 6GK5, 6HM5, 6HQ5, 6J4, 6J5, 6J5G, 6J5GT, 6K5G, 6K5GT, 6K5MG, 6L5G, 6P5G, 6S3P, 6S45P, 6SF5, 6T2, 7245, 7586, 76, 7895, 7A4, 7B4, 8556, 8627, 8628, 8808, 9002, A2521, A2599, CV5112, CV6, E20C, E88C, EC52, EC80, EC8010, EC8020, EC86, EC866, EC88, EC88C, EC90, EC92, EC97, NY67