Tube Replacement for Altis Reference DAC

I am using the Altis Centauri transport and Reference DAC (4 tube version). They are beautifully built and produce a wonderful sound. Could any of you who use the Reference DAC suggest the best place to buy tube replacements? Many thanks for your response.
I'm using some NOS German Fisher's with Bluenote Tube dampener's. Call Chris Kipp@ 916 447 2100.