I have not checked, but years ago I purchased Phenoseal from Player
Piano Company in Wichita, Kansas, and a large bottle has lasted me
through several pianos and does not seem to be exhausted.

If you cannot obtain Phenoseal, you might consider using something
a little more traditional: shellac.  Shellac is an excellent sealant
and is natural, removable, and is the original material used by the
suppliers.  I always coat the insides of pneumatics that I cover with
a good coat of shellac before recovering them.

When I was rebuilding my Ampico, it was recommended to me to pour
Phenoseal inside the stack before adding the pneumatics and valves;
it was easy (messy) but seems to have worked fine.  If you don't have
a large channel, however, it may be easier to simply paint with shellac.
(I always use shellac with a clearer conscience then I do with Phenoseal
or the like.)

Eliyahu Shahar