Caspers Electronics makes a knock gauge and lots of venders sell that same gauge. www.casperselectronics.com
Kirbans and Postons also sell it .Cost is around $70.
I like that one best because it tells you severity with the 10 leds and it's real time (not thru the ECM).
It reads from the knock sensor/controller. The A/F gives you O2 readings. Two very different things. I prefer a digital multimeter over a A/F gauge. (an EGT would b even better)