I don't know if "linear potentiometer" is the term for what I want. I wanted to determine the linear position of something in relation to something else (with more resolution than an optical encoder thing as described in the encoder tutorial), and did in fact find a bunch of linear position sensors online, but they are significantly more expensive than I thought they would be. I figured since there is a rotary potentiometer contained inside a 3 dollar servo that buying one alone would be, you know, less than 3 dollars, not 90 dollars.
Anyways, can somebody point me at something that does what I want? I don't know the correct electronic terms for anything, so I have a hard time googling for things. I would even be happy making my own; it seems like all you need is two wires and a length of some consistent resisting material and it should be easy to graph out a lookup table in not too much time for X resistance = X distance between contact points on the resisting material, correct?