As someone else pointed out, Static Drips (current build’s sprites) is an easy way to grab in-game sprites (could change once we switch to Unity).
As for the outlines, I myself use Paint.NET + plugins to generate an outline with a shadow around it. I go over it in my old video (timestamped). Basically you take the tiny 8x8/16x16 sprite, increase canvas size so there is space around it, then scale it up by the desired amount (400%, 500%, etc) and lastly you apply the outline effects.
I might make a new video about just this process alone though, since multiple people want to know how to do it and I cant blame them for not wanting to sit through my dated crap lol
Edit: Made a video on what I just wrote https://www.youtube.com/watch?v=xab6NDG6YcY