Most searches for those two lead to Ao3 or FanFiction.net, but the real trick is filtering. Tagging on Ao3 is your strongest tool – I'd start with 'Mako/Korra', obviously, but then add 'Slow Burn', 'Angst', 'Emotional Hurt/Comfort', and maybe 'Post-Canon'. That usually weeds out the shorter, fluffier stuff.
I found this one author, BlueFireDreams, who writes almost exclusively for that pairing with a focus on political tension and personal regret after the series. Their work 'Ashes in the Wind' is over 200k and spends chapters just on their awkward, painful attempts at conversation years later. It's less about romance and more about the emotional debris they left each other, which honestly feels more true to their characters anyway.
The FFN app can be weird for filtering, but sorting by word count and then skimming summaries for keywords like 'reconciliation' or 'regret' can turn up some older, massive fics that might have flown under the radar on newer platforms. A lot of the deepest ones seem to thrive on the fact their relationship in the show was such a messy foundation to build from.