The repository is available at https://github.com/realxtend/
Build scripts exist for Windows (VS2010-2015 supported) and Ubuntu Linux. Compiling for Android is also possible. However, at this early stage there are no prebuilt binaries available.
For those unfamiliar with the Tundra-Urho3D project, it's a lightweight reimplementation of the realXtend Tundra 2.x.x core without the heavy libraries Qt and Ogre. Instead the MIT licensed, multiplatform Urho3D (urho3d.github.io) game engine is used as the base framework and as the renderer.
Thanks: Lasse Öörni of Ludocraft