You can make one...details in my how to in the technical archive....
Failng that, it is possible to get an 11mm ring end spanner on one of the pulley retaining bolts, then using another spanner looped over the open end of the 11mm to get better leverage, then turn the shaft so the shank of the spanner is resting against the fan nut.....this will lock the water pump in place for you to undo....remeber the fan is a left hand thread.