Hi Bronson,
Ask and ye shall receive! As you've already found, iFixit doesn't have a guide for you, but there are a lot of phones out there and they can't do everything. Fortunately, there are other sites who share iFixit's vision, and the Spanish site Nadie Me Llama Gallina has just what you're looking for.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/xiaomi-191/xiaomi-redmi-note-9s-1228/pulsadores-laterales-10972|Manuales / Xiaomi Redmi Note 9S / Side push buttons|new_window=true]
You can find replacement buttons on the usual sites like eBay and AliExpress. Here's an example.
[link|https://www.aliexpress.us/item/3256801444887006.html|New Power Volume Side Key Button Flex Cable Ribbon For Xiaomi Redmi Note 9S 9 Pro Replacement Parts|new_window=true]
Good luck; let us know how it goes!