Terracotta can be downloaded from the Terracotta website under the download section.

Servoy integrates with the Open Source edition of Terracotta and is tested against the "Terracotta 3.7.0 (including Ehcache 2.6.0 and Quartz 2.1.6)" release. The integration should remain functional when new versions of Terracotta become available, as long as tim-api.jar remains 1.3.0 compatible.

The following link is a direct link to the download of the "Terracotta 3.7.0 (including Ehcache 2.6.0 and Quartz 2.1.6)" cross platform binary: Click here

Terracotta 3.5.0 introduced a bug that prevented it from working with Servoy. That bug was fixed in Terracotta 3.7.0.
So please use Terracotta 3.7.0 or later. If you need older versions try Terracotta 3.1.1 - 3.4.1.