1/4 inch or 3/8 should do...
I'm sure Hunter will mail them to you. For more info, try: customerservice@huntermarine.comor Call (800) 771-5556.Also, there is a small gasket on the top where you screw down the valve, and that needs to be replaced periodically, too.To make your own, get something somewhat pliable, like neoprene (wetsuite material) would work well.Fair winds, Tom