Fighter5347 has suggest some good sites to visit for free and paid models. If you’re looking for specific models you may find yourself out of luck when it comes to what’s included. If you don’t mind what models you’re getting, I can tell you that if you ‘Migrate’](A new, community-hosted Unreal Engine Wiki - Announcements and Releases - Unreal Engine Forums) assets from our Content Examples you’ll get a plethora of assets to get you started. You can do this for all the other content examples as well. The only things you would have trouble migrating would be anything that is tied to any C++ code as it will migrate the Blueprint and the asset but not the C++ code that drives it. An example of that would be the weapon from Shooter Game or the Minion walker character from the Strategy game. Migrating the assets like walls, floors, textures and such shouldn’t be a problem at all!
I hope this helps you get something going.
If you want feedback on your level once you get it started or any help feel free to post your questions here and the level you’re working on in the WIP section.