Depending on a given game's releases, horizontal arts may be hard to come by. CD/DVD games, for example, I have yet to see a second game with horizontal cover, and if you like Japanese covers for SNES games, cover orientation was all over the place.

Still, you can try to check for a given game on Gamefaqs, or if it made its way into either GOG or Steam, both of which having emulated titles. Wikia/Fandom may also have something. And I remember having seen other cover arts sources some years back, but I can't remember their names.

But for the ones I mentioned, for Gamefaqs, you can find the box arts under Media > Boxes. For GOG, you can find the images either on your library (if you have the game), or on GOGDB (a project similar to SteamDB), and both of which require getting the link for the image, and omitting the _196. part before downloading (and optionally, also swap the extension in the link for .png). For Steam, the image should be accessible in the game's store page.