The Aircrack-ng community has maintained a "Legacy Driver Pack" for over a decade. This is the most stable source for Windows 7/8/10.
The user morrownr maintains the definitive USB Wi-Fi adapter guide. For RTL8812AU (802.11ac, but backward compatible with n): download alfa network 80211n driver patched
Alfa Network provides basic WHQL-certified drivers on their official CD. These are fine for a home user who just wants to connect to a Wi-Fi network. However, for advanced operations, the stock drivers have limitations: The Aircrack-ng community has maintained a "Legacy Driver
The ath9k_htc driver is already patched for injection, but it is disabled by default. for advanced operations