I am especially grateful for this question because it's not often (actually it's never) that I come in contact with someone other than my dad who appreciates the lesser known magical creatures! In fact, most people deny their existence (as I'm sure you know from studying)! Anyway, my mum taught me all about nargles (did you know they are a type of fairy?) when I was a young girl, and luckily I still remember most of what she told me.
Now to answer your question. To put it simply, Nargles are mainly found in places where there is more magic. At Hogwarts they are everywhere because there are so many magical people in one place. Same with Diagon Alley. They feed off of that magical air, if you know what I mean. It's not unheard of to see some in the Muggle world, but they generally think Muggles are boring folk. You see, nargles are playful and mischievous, so they seek mayhem. But they are also very pesky, which is why I wear my butterbeer necklace and radish earrings to keep them away.
And a last thing. Nargles are only seen by people who believe they exist. You can see them best when wearing spectrespecs glasses.
Wow! I could go on and on, but I think I answered your question, so I should stop. I just love spreading the knowledge of incredible magical beings!
Have a lovely day, and watch out for the nargles, Eden!