You could buy a 3.3v relay board to replace the 5v board that you have. That SHOULD work okay, but bear in mind that the voltage regulator on the NodeMCU board is stepping-down the USB power and you'd be asking it to drive the board and the relay. If you final solution was one where the relay will only normally be energised for a short period of time (such as operating outside lights that will stay on for a couple of minutes then go off) then a 3.3v relay powered directly off the NodeMCU pins would be fine.
Otherwise, I'd suggest using a power supply similar to the one you're using at present. This could be wired to power the NodeMCU board from 3.3v and then drive either a 3.3v or 5v relay depending on what you prefer. If you were going down this route you wouldn't need the external USB supply to power the NodeMCU.
I know I mentioned this before, but have you looked at the Sonoff switches? You can buy one from China for less than £5 delivered and they have an Esp8266 and relay built-in and don't need an external power supply. You have to connect mains Line and Neutral to them, so in UK wiring at least you'd need to locate it near the ceiling rose for the light, but you'd have similar issues picking-up- a supply to power your home-made device.
You have to solder a 5 pin header onto the board, but this is very easy. If you want an external push-button switch (there's one on the Sonoff device, but if you're putting it in a ceiling cavity then it wont be accessible) then you run this from two of the pins on the header once you've flashed the code onto the Sonoff.
You need a cheap FTDI programmer to flash the code, but once again these are very cheap.
If you do choose to take a look at the Sonoffs then NEVER connect it to the computer and the mains supply at the same time. You will kill the computer, and probably yourself at the same time. On that subject, be very careful what you do next when you start connecting mains power to your existing setup. Check that everything is connected-up correctly before plugging-in to the mains and make sure that there's no chance of the mains and the low-voltage electronics coming in to contact with each other while you're messing around with it on the bench.