First off, welcome to the forums!

You're free to introduce yourself in the Introductions sub-forum if you want.
I don't know if a spritesheet like that exists. But it's a rather simple thing, actually. Each sprite of all five characters in SMBX uses up a space of 100x100 pixels each in the spritesheets. With that information, you can make a graphic with the size 1000x1000 where each 100x100 square has, for example, a different color than the squares nearby. That's about equal to the spritesheet you were talking about.
Hope this helps.
