Use the search users interested in direct swaps feature to find other members that want to participate in direct swaps. Make sure that you select the option for direct swaps and then use the other fields to narrow down the list.

Do not contact members that have indicated they are not interested in direct swaps.

The easiest way to find swap partners is to use the Postcrossing Forum. The forum has a very active swapping community. On the forum, swaps are also referred to as trades. Find the appropriate trade section and read through the message threads or create your own.