In order to buy rare MOC's I find that my best way is through other collectors who know other collectors etc, when you build up contacts in this game it is worth its weight in gold, the majority of rare items I have got has come through contacts and people helping me out as I've done with them.
When I got my Palitoy 30 back Yoda, AFA 80, late on last year I had no idea about it until a friend I know in collecting was good enough to inform me as he knew I needed that figure to complete my Palitoy 30 back B graded run, going on ebay etc is fine but nothing much comes up, only from time to time and every man and his dog knows about it.
So, where do you buy rare Moc's, my answer would be from other collectors who decide to sell and inform you, this hobby is about building friendships and trust but most importantly having a good and fair reputation, thankfully I have and that's why people help me along the way.