Best answer: You can buy a replacement S Pen directly from Samsung in blue or purple for $15, but third parties sell them for a bit cheaper and in more colors.
Samsung: ($15)
Can you just swap out your S Pen?
If your S Pen goes missing or you've just decided you want to try a different color, there's no better place to buy something than directly from the source. You know you're getting exactly what you're paying for, and should something happen to go wrong, you only have to deal with one company to find a solution.
Samsung has the S Pen in blue (actually yellow?), black, purple, and brown for $15. Once you receive your S Pen, just slide it into the Note 9's receptacle to pair the two devices together.
Be wary of cheap knockoffs
As with any product, there are bound to be third-party options available for cheaper that claim to work flawlessly, but don't be fooled too easily. The S Pen for the Galaxy Note 9 has more nuanced features than ever before that third parties simply can't replicate.
That doesn't mean there aren't good outside sources to buy a legitimate S Pen from. Mobile Fun lists the official Samsung S Pen on its site for $25, the Note 9 is offered in globally.
What about S Pens from older phones?
If you have an older S Pen sitting around from a phone like the Galaxy Note 8, it'll technically work with the Note 9, and it'll even fit in the phone, but it won't have the super-capacitor inside the Note 9's S Pen that allows for its new wireless capabilities. Plus, with the Note 9 S Pen sitting at only $15, there's no point in spending $5 more on the older model. Does not compute.
Make your phone whole again
The S Pen is one of the Galaxy Note 9's most crucial features, so be sure to keep one handy at all times.
Our pick
Galaxy Note 9 S Pen
Do your Note 9 right with a proper S Pen.
Don't get a third-party stylus — buy a genuine S Pen replacement and enjoy the Note 9 as it was intended. Get it for $15 in any of the four colors (and remember that, for some reason, "Ocean Blue" is actually yellow).