Matt,
Parts Connexion have these Rikens, and they might even be on 'close-out' at around $US2 apiece.
This series resistor (27K I believe you found?) is CRUCIAL for sonics. The signal passes through it to the tube, whereas in shunt mode the pot actually bypasses the signal and any nasties it confers are isolated to the wiper, which happily sits at ground and thus far from the actual signal path.
The resistor needs to be non-inductive - this is important. Many metal films have an etched spiral winding on ceramic, and this is, in fact, inductive and not advisable in this application. The carbon comps are good, but must be quality, like the Allen Bradley stuff. Beyschlag is very good, quite transparent, but a tad dry. The Rikens add a little distortion, all low order, and have a sweet sound. Some have found that it's too much, so you simply use a Riken of 10K and a Beyschlag of 7K (NPV 6K8) in series.
You choose - the joy of DIY!!
Hugh