Contact NZXT. They DO sell some replacement parts.
You should actually be able to replace JUST the switch itself, without the button. If you pull the switch you will probably find that it is a common, likely universal, PC power on switch. Most of them have some kind of part number on them and THAT will usually lead you to a replacement.
Not always though. Occasionally the power switch is proprietary. Seems I read about a whole bunch of these Switch 810 (And other NZXT case power on switches) going bad and that there were universal replacements for them on Amazon and Ebay. Need to know what the actual switch itself looks like though with out the button or get a part number off of it.
You could also just get a universal switch and wire it up, mount it UNDER the case, or wherever it's out of sight, and use that to trigger the unit on or off. Might even be able to pop it directly in place of the one that's there now.
https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=PC+power+switch