[ Nathan Woodhull wrote in 130101 MMDigest, "I make them out of
 [ polyethylene from old coffee can lids and they seem to work well."

For those who do not know their plastics, polyethylene is a very stable
and long lasting material and is likely a very good choice for Duo-Art
valve leather retainers.  It is also less likely to damage the stems by
biting into them as do the original linen phenolic material.  There also
might be some ready-made nylon or other washers available.  Running a
reamer (available in just about every size imaginable) through a stock
washer could yield just the right size.

Some stems are looser than others.  I have had to use a dot of glue to
hold the original retainers.  If the inside diameter (I.D.) is known it
can be looked up at McMaster-Carr Supply Company.  Of course, they could
be an odd size; a search through a hardware store that has lots of the
boxes of odd washers, etc., might also turn something up.  I also have
a punch press and would probably just make them myself.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/