Technically still on time since it's 2nd christmas day here for another hour or so
Last year i turned a string of very sketchy Aliexpress incandescent christmas lights into a string of slightly less sketchy (only slightly) neon NE2 lights, using standard, green and blue NE-2 indicators. Turns out, the little indicator bulbs fit into the standard lamp sockets just fine.
Sadly the green ones are of terrible quality - within weeks of use they lose most of their light output.
If you're gonna build this yourself, be aware that those little lamp sockets are not meant to carry 1kV or so. The current is low, the danger of the shock you could get is very small, but any arcing could turn a christmas tree into a bonfire with ease!
If you have parallel style christmas lights (simply not a thing here), it's safer to give each bulb its own resistor and run them straight from mains.
https://youtu.be/MTaQ96BH5q8