I greatly increased and diversified my trader supply by participating in round robins. I always make a point of picking up 15 traders from every casino i went to. Then i trade traders for traders quite often which has made my selection much much bigger. So this way, even if i dont find anything that i need for my collection in the round robin, i can still benefit.

Of course the fact that i travel so much has helped give me a pretty good head start in the trader department.