Anything will do really, the smaller the better, so if you have one left over from a donor etc.......
Cheers.
Pete.

I used a speed controller on a heater motor - http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=170651992519

A 5amp controller would have probably been better as the control knob is quite sensitive (my heater is 35watts).

http://www.ebay.co.uk/itm/12V-24V-3A-DC-Motor-Speed-Control-PWM-HHO-RC-Controller-UK-FAST-DISPATCH-/181093105172?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item2a29fd7e14

[Edited on 6/3/13 by inkafone]