For asset creation Blender is by far the best choice. It is free and open source and it recently got an overhaul which made it even better. There are also tons of tutorials on how to use it. Check it out here.
As for ready-made 3d content, a simple online search yields quite a few options, such as https://free3d.com/
There are numerous free assets out there. You should be able to find what fits your needs best.