First place I would look is in the Learn tab and Marketplace tab of the Launcher, there are quite a few free projects and examples that include a LOT of content, plus you are free to use any of these assets in a commercially released game. Each of the sample game contains a lot of assets (meshes, materials, textures, sounds and more). Here are some of the best ones:
Marketplace Tab of the Launcher:
- Mixamo Character Pack - 15 free characters with some animations ready to be used
- Animation Starter Pack - a bunch of animations you can use for characters, can be re-targeted to the Mixamo pack
- GameTextures Material Pack
- Platformer Game
- Landscape Mountains
- Elemental Demo
- Vehicle Game
Learn Tab of the Launcher:
- Content Examples
- Particle
- Blueprints
- First Person Shooter
- Tower Defense
- Matinee
I highly recommend looking through the above examples, a lot of great content.
Beyond that there are sites like
- TurboSquid (sort the products by price and you will find many free ones) - :///
- TF3DM lots of models ://tf3dm/
- Archive3D ://archive3d.net/
Some of the models might not be in FBX format, but you can open them up with Blender (or 3dsMax/Maya etc) and then export them out as FBX files.
Hope that helps! Welcome to the forums. 