Where can I get a Fremen stillsuit? I'm looking for a functional unit,
so no movie props or silly answers like "I saw one in a market on
Arakis".

Failing that, is there a portable device that can reclaim water from
expired air?

Exhale through a silica gel column, then recover the water by heating
and condensation.

This is a good starting point

Google: "water loss" respiratory "water recovery"

Found:

A prototype military stillsuit
http://www.asc2002.com/Abstracts_only/k/KA-01.pdf

Biology and technical aspects
http://www.cmnh.org/dinoarch/1997Jan/msg00243.html

How birds do it
http://www.usd.edu/biol/faculty/swanson/ecophys/readings/tieleman.doc

Hey, thanks xarqi-ga.  This is similar to Mesosystems Technologies'
device, but it does it without an energy source, recovering water
through a membrane system

Alternatively, biology has answered the problem without heating too -
a crane's nares work through premoistening intake air with exhales
moisture.  It doesn't eliminate water loss, but it saves a lot and is
more comfortable (lower pressure drop) than Mesosystems Technologies'
mask.

My question is more "Where can a _get_ one?"  Mesosystems
Technologies' product is at the mockup/ prototype stage and I don't
have a beak.

Need an energy source?  How about the big one - the Sun!  Just rig up
a solar oven - it would only need to be small.  Bake the moisture out
of the silica gel that way.  I figure that if you need such a device,
it will probably be sunny where you are.

Two other vaguely related things:
1) Build a dew catcher.  Dig a hole in the evening; place a dish in
the middle of it; cover the hole with a plastic sheet; pierce the
centre of the sheet, and weigh the centre down with a small stone.

2) Urine recycler.  Just the same except - supply your own moisture
into the hole first, and don't pierce the plastic.  You end up
drinking distilled water!