You will find here and there some free materials… But in my opinion you will need to purchase some good stuff.
I personally work a lot with Substance Designer and Painter, they offer alreday a large and good looking number of materials for free.
You can also create your own textures with Substnace Designer or simply download materials from users through Substnace share service.
Otherwise you can puchase some photoscanned textures from the marketplace like the real surfaces packs…
You can find good wood textures here Arroway Textures – Professional Textures
Textures.com has also a very large database for a very low price but the textures are not PBR ready…
evermotion.org offers sometoimes good and free materials
The last step would be to create your own textures by photogrammetry… You can find some good tutorials from epic here on the site…