If it were my amp, I'd replace them all, with every stage matched as closely as possible, with the best NOS tubes that are appropriate for the amp. Keep the others from the original set as "emergency spares". Make sure the tubes are low in microphonics, because preamps are the place that is very sensitive to microphonic tubes.
You can try Audio Research for spares, but they won't be NOS. Some amp makers don't recommend NOS tubes for their amps. Vacuum Tube Valley, Vintage Tube Services, Upscale Audio, Angela Audio, etc. are some places to find tubes.
You can try Audio Research for spares, but they won't be NOS. Some amp makers don't recommend NOS tubes for their amps. Vacuum Tube Valley, Vintage Tube Services, Upscale Audio, Angela Audio, etc. are some places to find tubes.