We used them at the Petersen Museum in LA.  Found a half-dozen vendors in
the Yellow Pages under Special Effects, likely a local phenomenon.

There are oil and wafer based systems.  The latter have zero
environmental/conservation implications.  There's dozens of stock scents.
The wafer has air blown across it and the scent is directed by flexible
tubing.

In surveys, most of the visitors who professed conscious awareness of the
smells thought they came from the snack bar.  That explained the apple-pie
scent in the bungalow exhibit and the greasy burger smell by the
restaurant exhibit.  But we also had a burning motor oil and hot-rubber
smell in a racing exhibit, and I wonder what the visitors thought about
the snack bar after smelling that one.

As to those whose awareness remained subliminal, alas, that fell beyond
our survey methodology.

Matt Roth