If you're just needing a stylus for normal touch-input, then any passive stylus should work fine. However, if you also want it for handwriting features or anything beyond basic touch-input, then you'll need one of the Surface Pens as they're the only styluses compatible with the Surface Duo. The standard Surface Pen, the new Slim one, and even the big Hub 2 Pen all work, and they can usually be found on eBay for around $35.

Sorry for replying twice, my server glitch out for a second and I accidentally posted the same reply twice. 😅

I've been using the original Pixel Fold for almost a year now, and so far it's held up really well. The build quality hasn't degraded any despite daily use, and it's even held up surprisingly well against abuse.

I've fumbled the device on more than one occasion, and even once dropped it straight down on hard tile from about five feet, but surprisingly it came out just fine without a crack, dent, or scuff in sight. Granted, I do have a case on my fold since I'm accident prone, but I've not had any issues at all with the durability of the folding display.

However, I would still recommend getting Preferred Care or some other protection plan in addition to the standard warranty, because if the screen does break, and it's not covered by the standard warranty, you're kind of on your own. And it's real expensive to get them repaired.

The Pixel Fold sort of falls into the mini tablet category, albeit significantly more expensive.