The following site is a useful resource:http://www.orselli.net/sources.htm
Its called the Great Big Exhibit Resource List, but many of the vendors are useful theatrically as well. I suppose that makes sense as you could look at it as theatre scenery is an exhibit of sorts. They are a variety of fake foods listed, some fake grass places, electrical controls, plus these string pots: http://www.stringpot.com/.
I haven't really used anything line this previously, but it seems cool. What jumps into my head would be tracking something like a line set where there isn't a shaft to use a standard encoder with.