Farm auctions are usually a good place to pick up some old working equipment that has been used in the past and just needs some WD-40 and some grease and would work just as good as something that would cost you twice as much. You might even be able get a squeeze chute.
Keep your eye open for an add in the farm section of your classifieds in the newspaper. I've seen equipment like this offered before and it usually does not last long. If priced appropriately, usually will sell the first day or two. A lot of people look for used equipment but not many people sell it because they use it forever and ever.
If you really need one now I'd say just go out and buy a new one. They really aren't that expensive and you know that the new one will in fact work and won't be severly rusted out, bent or have other damage. Preifert makes a good headgate --- I haven't priced one in a long time but I bet you can get one in the $400 range.
If you have the luxury of time, then keep on trying to find a GOOD used one --- but first find out what new ones cost. Many's the time that I have observed people bidding on things at auctions and essentially paying retail prices for pretty well used and worn stuff, sometimes even in excess of retail!
I have been to a couple of auctions this spring and old used beat up chutes and head gates have been selling for near new price. For the money I have seen them sell for you would be better off to buy new. In fact I have thought about hauling my old one to a sale and replacing it with a new one with just a little money thrown in.