Google Brent Jessee or Vintage Tube Services. Both are respected tube sellers who will help you find the tubes you want. They probably will not be cheap , but who knows, you may get lucky. Buying tubes from these guys is not cheap but it is cheaper than learning about NOS tubes by trial and error. Brent will usually get the tubes to you faster than Andy (VTS).
As petg60 has recommended, get familiar with the sound of your DAC/amp with the stock tubes in it and have an idea of how you would like to change that sound with your new tubes before calling the tube sellers..