I'm guessing based off your post you want an internal adapter, and not a USB one. I'm a little surprised you want one at all given it already has an internal card, but hey–if you want a bit of an upgrade, here's your card.
As @SiXandSeven8ths said, you'll need a card with a low profile bracket. Luckily, almost every card comes with one–this one included!
Since you're working in a proprietary case and weird PSU, I decided against any cards with an external power connector. I also went with wireless AC, since you seem to be looking for an upgrade. This left the TP-Link Archer T6E AC1300, TRENDnet TEW-807ECH, Asus PCE-AC56, and our Rosewill card all around the same price point. Both the TP-Link and Asus cards seem to have massive compatibility issues with Windows 10 Fall Creator's Update, and the TRENDnet card seems to have some QC issues, leaving the Rosewill card.