Taylor Merchant sold the inexpensive self contained plastic viewers that
you can simply remove the lenses and install into your own viewer housing.
I have no idea if they are still in business or how to get in touch with
them however.
Reel-3D should be able to help you out otherwise:
Reel 3D
P.O. Box 2368
Culver City, CA. 90231
(213) 837-2368
A Photographers Place, the bookstore in New York, sold plastic stereo lens
pairs a few years ago also.
Additionally, you might contact Lorran Meares, the photographer for the
Sierra Club calendar of sacred places a few years ago, who photographs most
of his light-painted scenes in stereo, and who has installed a number of
shows using a roomful of Holmes type stereo viewers. He has just moved to
Santa Fe:
LorranM@aol.com
Hope this helps.
Sam