I owned LS-15 long time ago. Used it with McCormack DNA-225 amp. I remember liking the EH Gold Pin 6922 tubes in it. You can also try Gold Lion 6922.
For new production tubes I recommend either Tube Depot or Upscale Audio.
If you want to try NOS, email Brent Jesse, tell him what type of sound you are looking for and provide your budget and preamp model. I purchased from Brent before and he is solid.
One more important note - get matched pairs.