There will be loads on ebay nearer the time.
Exhibitor passes have to be purchased at the time you put your entries in. If you know someone who has not entered yet, that is the most cost effective way to go to HOYS if you are going for 2 or 3 days although you are not guaranteed a seat when it is busy.
i brought mine over a month ago now and they were limited numbers remaining. I couldn't get the seats I wanted that was for sure. Glad i booked mine early as going for the full day on sat to watch my old pony compete in the morning.
only suggestion is keep ringing and keep an eye on ebay.