As long as is metal geared to last and fast like @6v 0.1sec (ish) you will be fine, I use savox and ace ds1015 both have been fine after my years of punishment. Everyone has the preferred make or model.
Try putting a wanted add on here in trade section, lots of us have spare ones in box
example:
http://www.oople.com/forums/showthread.php?t=101635