I haven't got one of these, but I would assume they use a standard pitch and length screw - you should be able to take one (or the head/body if desperate) to a local screw supplier - in Coffs this is Eastland Trade Supplies for example
I lost the little sealer bit in a bottle filler a couple of weeks ago - quite screwed the day (aha.....)
I actually spent 10 mins looking on the lawn where I am pretty sure I lost it when cleaning stuff before mumbling 'something' and going to the shop the next day for another one
On the up side it prompted me to change to using a hose between the tap and the filler, which was pretty sweet.
Plenty on eBay, 8-32 UNC 3/8 is the size you want. I've bought the slightly longer 1/2 inch version from FasterKitsOnline (http://stores.ebay.com.au/fastenerkitsonline?_rdc=1) previously to mount the 809-HS on a bracket and received good services and prices.