If not eBay, I usually shop one of our OGR sponsors, or toytrain.info, Ray Ellen at trainmarket.com, Walt Hiteshew at toytrainsandguides.com and all of the facebook and yahoo for sale pages such as the AvailableOgauge yahoo page.
I also watch Stout and Ralston auctions and Dakota Paul. When all else fails I email Papa Eastman. :=}