You're looking at this the wrong way. Trying to find a swap is relatively pointless. Let's say you actually find someone who is interested in swapping. What's to say that the PD's of the two programs will agree? I probably wouldn't -- if one of my residents wants to leave, then they are free to leave. But I get to choose whom I fill that spot with, and it's unlikely to be some random person who you found on the internet who wants to swap.
So, you should just look for an open spot somewhere. If you find one, and they take you, then you resign your current spot.