First of all---congratulations on taking the step towards your credentialing. I sat for my CPTD last year and do not regret it at all.
I did my study prep with the Ann Arbor chapter of ATD. Here's a link to their page. I also heard great things about the ATD Kansas City study group that @Jennifer Witebsky references below (here's their page). @Sally Koppy could give you more information about their group.
You also have an amazing network of new friends through this community that have achieved their APTD and CPTD credentials. We are all here to support you!