welcome to the forum homefarm ,
i dare say you'll see them come up on e-bay , but the prices can go from bargain to unbelieveable, another way is to put up what you want in the 'for sale & wanted' section on the forum.
As well as collectors ,there are buyers & sellers & dealers here, even udimore is a half decent chap ,well seeing as he's lost so much weight he really is half the man he used to be :D :D
if these people haven't got it , they'll know someone who has
there is a links page here also, directing you to stockists that maybe able to help, the model shows are also a good place to be for what you want , you'll see when these are on in the 'farm related events' & if you can get to one , you'll be sure to get what you want
if you want help with anything , just ask :) :)