Joysticks/rudders use "Linear" type potentiomer, the original resistance you measure with multimeter.
Is probable this (picture from
Saitek.ru, showing pot' of X-52 twist rudder):
http://i17.photobucket.com/albums/b68/Bucic/DCSforum/controllers/saitek_potentiometer2.jpgYou could buy a better, example Bourns, Spectrol, but the problem is find one with exaclty same physical dimensions of axis shaft, angle of rotation (65º on above)...
The only things that really matter is resistance (in Kb) and type, "linear" in case, and degrees. You dont need a "original" one - which is usually the cheapest possible...
In this video the guy replace the original pot' of X-52 for another - probable Bourns.
http://www.youtube.com/watch?v=29AqO-BKTkcThe vide show a 32º pot:
http://i1.ytimg.com/vi/29AqO-BKTkc/maxresdefault.jpgSokol1