I recently bought 5 year old the Ezzy rig package. Good for beginners. Includes 2 sails and the whole rig, right down to the foot/extension ...thingy.
www.ezzy.com/sails/kids-rigs/Circa $500 for the small rig.
http://www.surfsailaustralia.com.au/listProduct/Windsurfing/Sails+Kids+and+YouthI couldn't find any second hand gear either.