You can do this for free. You just need to see in many webmaster communities people who ask for post exchanges. Visit their sites and see if you're pleased with the way they seem to handle the posting. Then just send a nice PM. It works and it's for free. Not to mention you already know a bit more about the ones that will post in your new community.
I did that, and regretted it. I'd much rather use a paid service, even considering how weak their services are.
The people I did post exchanges with posted terribly. One guy just copied posts and other copyrighted materials in order to create his posts. Another guy -- whom I thankfully only did a 10-post exchange with -- just went into the general forum and posted "that's funny!" or "wow!" to 10 topics. He was so obviously there only because he had to be. In the end, I ended up deleting all their posts, and I regretted ever doing it.
I'm sure there are good guys out there to do post exchanges with. I just didn't find them.