When I started my cabinet build project, I couldn't really find much in the way of correctly-sized marquee images for the 21:9 LG Ultrawide monitor I used as a marquee display... so I made my own.
These are marquee images for use for various console systems - GBA / Sega Genesis / NES / SNES / N64.
I've also included a 'template' png file in the .zip for each of these that could be used to make your own marquees if you want, for games I don't have included here. You can use GIMP / Photoshop / Whatever image editing software you want, and overlay the box art for your game on the template. Here are the box image sizes for use with each template (pixel width x height:)
NES Box art size: 219 x 300
SNES Box art size: 360 x 257
N64 Box art size: 360 x 263
GBA Box art size: 320 x 320
Genesis Box art size: 224 x 320
I found the box art I used linked over on Reddit.