How to find bounding rectangles for sprites in a texture at runtime?
Take a look here Unity - Scripting API: Sprite.
Shows Sprite.rect.
Chris-Trueman, the texture is loaded from the file system at runtime
Then you will need to define it yourself. If its a sprite sheet you may want to have some data about that sprite sheet loaded as well when the texture is loaded, so you can use that data to create the sprites.
how to does it Sprite Editor in Automatic slice mode? are there any sources or libraries?
Its built into the editor. You would have to take a look and see what you can find for it.