Just head down to the Market section of the boards and make a post saying which cards you're looking for and what you have available for trade. A lot of people don't post everything they have, just a few of their more interesting cards or the ones from newer sets. If you're looking for more valuable stuff than you can offer, you can often buy cards from people for less than TLG/YTG prices. Once you work something out with someone, you'll mail your cards to each other.
