Assuming you have a curtis controller...
from curtis:
Quote:
Speed Sensor
A speed sensor is required for use with any 1266 controller. The speed sensor must be of a pulse type, and must interface to the controller with an open collector NPN transistor output. The most common sensor type will be a Hall effect switch IC, such as the Allegro type UGN3132 or Micro Switch type SS11; these work with an eight-pole (four pulses per revolution) ring magnet attached to the motor shaft.
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The UGN3132 has been discontinued. The direct replacement from Allegro is the A1203. You want the UA package as the LH is surface mount.
$1.55 from Digikey 
The one linked above from sparkfun should work as well.
Cheers,
-sj