Hi mediamatchstick,
You can download previous versions of Events Calendar PRO on your account’s downloads page (https://theeventscalendar.com/my-account/downloads/). Be sure to also run it alongside The Events Calendar 3.6.1, which you can download from the WordPress Plugin Directory here. Those should get you all set to run 3.6.1 🙂
Do please note that we can only provide limited support for people running outdated versions of the plugin, and we do always recommend that you stay up-to-date with the latest version. I understand that your theme was designed to work with 3.6.1, but just keep that in mind if you need support in the future.
Best,
Leah