• Unreal Marketplace
  Its getting bigger and bigger every day.
  Don’t forget about Epic examples! You can use assets from them as well!
• Unity Asset Store
  Currently biggest asset store for game developers. If you are buying 3d assets ask creator if package contains .FBX files. I see that new packages have Unity binary format instead of .FBX files that you can import to 3d program and export to UE4.
• Turbosquid
  Focused on graphics only, but lot of assets!
• GameDevMarket
  Lot’s of 2d assets.
• 3dmodels-textures.com
• http://opengameart.org
• CG Trader
• CreativeMarket

If you found new Asset Store please let me know!