I have the same question: https://dietpi.com/forum/t/good-wifi-5-802-11ac-usb-adapter/6201/1
But if you need it for RPi 4 specifically, I recommend to ask on the RPi forum. The DietPi version is not relevant, only the hardware, firmware/kernel version, which is the same as with RPi OS. Here one example which you shouldn’t buy at time of writing: https://forums.raspberrypi.com/viewtopic.php?t=330500