Well I'm an outliner and a pantser, so I'll go ahead and answer.
I'm always thinking about my WIP and running through scenes in my head while I'm driving to work, prepping dinner, falling asleep at night. So it isn't hard when I sit down to write to come up with the, "what next?" ...most of the time!
Which is why I do sometimes use an outline, especially for the longer, more complex pieces.
With or without an outline, I always try to write from the first word all the way to the last without editing anything. So, the first draft is usually one hot mess, and the edit/revise stage can be challenging, but when I used to edit as I went along, I never finished anything. So I'll sit down, review what I wrote last just to refresh my memory, or to read whatever note I left myself, and then just start typing. When I don't have an outline, it's because the story just comes to me pretty easily, and the characters just do their thing and I'm essentially along for the ride to record their history.
As far as where my ideas come from, that would be everywhere! A conversation I overheard at the grocery store between strangers, a book I'm reading on quantum mechanics, my boss' smart as comment, whatever. Sometimes, I can't even remember where an idea came from, I'll just find a scribbled note in my purse in my handwriting with a few sentences on it about a character or a place, and it turns into my next short story! The ones I write to myself in the middle of the night without turning the light on usually just get tossed, though, cause even I can't read them. I'm always a little sad when I have to do that.