Airplane & Heli speed cons are great, but I would think lack reverse...
Any inexpensive car speed con. will likely fit the bill, but there would be voltage limits.

here is an example of about the cheepest fwd/rev I know of other than used.

http://www3.towerhobbies.com/cgi-bin...?&I=LXAMN7&P=0

You'll should check locally if you're not from this side of the pond.