You must be certain your transplant center will accept a donor found through the internet before going forward. Many will not accept a stranger.
We met Jenna's donor on httpp://livingdonorsonline.org message forum - you can post a new thread in the "Looking for" section - it's free - give a small summary of your situation, include your location and blood type. I used a different yahoo email so I knew exactly why where the contacts were coming from - got many from overseas and some unusual offers, but in the end I met some really generous souls and it ultimately lead to Patrice donating to Jenna.
Matchingdonorsonline.com charges a fee, but there are lots of participants so you might have better luck. They claim to have financial aid available.
http://www.paireddonation.org/ Alliance for Paired Donation helps arrange swaps, but since they have no partners here in Califonia I don't have any experience with them.
There's also
http://www.kidneyregistry.org/ National Kidney Registry. They help swaps happen, so even if you have a willing incompatible donor they will help find a kidney for you to exchange. They charge a lot but they also have financial aid available. There is a section of their website devoted to finding a donor
http://www.kidneyregistry.org/no_donor.php