Here is the
Alps (5€) and a cheaper
alternative (~2€) that performed equally, uhmm, bad
Hooking them up to a teensy is really simple, but they really need some dedicated calibration logic or a rather large central dead-zone, as due to their resistive nature and springs the zero-position is not exact. Nothing that can not be solved, but a mouse replacement needing pixel-accurate precision is definitely different than a game controller usage - try to surf via PS3 browser, for example, I find it hardly bearable and always overshot the links.
What works well, though, is using 4 (or even 8 with diagonals) directions of the joystick as modifiers: I tried that too, it was ok, but the weak springs make the modifier stay on a lot longer than needed so it often actioned on the next keypress, too.
All in all my final verdict is: Nice and cheap addon, interesting use-cases possible but nothing compared to a true trackpoint.