Big electrical equipment, whatever it is, almost always comes with a plate or a sticker, with the electrical specifications printed on it. The attached picture is an example. It is a neon sign transformer I found in my junk collection, and its little plate says its primary is rated at 120 VAC, and its secondary is rated at 9000 VAC at 25 mA. Also notice that number 225 VA. It is 9000 V multiplied by 25 mA. I don't know how close this 9000V is to the "average" you are asking about. I am guessing any NST with a secondary in the range of 5000 to 15000 would work.