Encapsulated mercury switches are probably still available from
industrial sources but they can also be found in surplus junk fairly
easily.  Heavy duty old "silent" wall switches have disc shaped mercury
switches.  They have no leads as they fit between spring contacts, so
you need to solder flexible wire to them.

Many old pressure switches used the very heavy duty encapsulated
mercury switches mentioned by David Dewey.  I have a couple of
"Mercoid" pressure switches that have them beautifully displayed behind
glass covers.  These things were made into the 1970s, I believe, and
maybe even later, so they should be easier to find than 1920s washing
machine parts.

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