Well, one obvious difference is the medium. Manga is visual, so you can directly see the slime's appearance, the expressions of characters, and the action scenes. For example, in a slime battle scene, the manga can show the slime's movements vividly. However, a light novel uses words to describe all these. It might describe the slime as a gooey, translucent blob that pulsates with a strange energy.
Well, there are several options. Some people find it on Crunchyroll Manga or Comixology. But make sure to check for the official and licensed sources to support the creators.
The ending point of the Slime anime in the manga can vary. Sometimes the anime might catch up to a certain point in the manga storyline, but it's not always a straightforward match. It could also stop before reaching a major plot development.