https://www.twistedthrottle.com/denali-soundbomb-mini-electromagnetic-low-tone-motorcycle-horn

Anyone used this to upgrade their horn? Does it "plug n play" with the 2008 Ninja 250? Seems like a really inexpensive way to upgrade the stock horn to something that can actually be heard.

Not that one, but I put a FIAMM 72112 Freeway Blaster on mine, mostly plug and play, works fine! Just don't accidently hit the button in the garage.

Doesn't fit in the stock location, the one you linked most likely wouldn't either. Someone here came up with a good way to mount it that I copied, I'll see if I can find the thread.

Edit: Found it: https://www.ninjette.org/forums/showthread.php?t=106589