Not sure what you're asking for... linear position sensing is available using almost any discipline imaginable. Obviously linear pots do it resistively, LVDT's do it with inductance (and there are other inductive means too), there are linear position sensor based on capacitance, etc.
By "linear encoder" do you mean something with an inherently digital output, as opposed to analog? If that's what you meant, as noted by others you can derive that by A/Ding one of the aforementioned linear sensors. That would give you total control over number of bits, dynamic range, scaling, curve shaping, etc.
As you noted, there are industrial linear encoders but they're not "something in a ready to use package" for direct human input.
No matter what output data format you're seeking, your best and easiest solution for human input is likely going to be using something designed for human input in a "ready to use package" - which means a linear pot driving an A/D input through a buffer. Seriously, that's about as simple as it's going to get, both mechanically and electronically.
Hope this helps!