Front sensors are no longer available. Junk yards would be your only source. Rear sensors can be found on Ebay. I'm sure its limited availability.
The sensors can be tested using a multimeter. There is a chart floating around somewhere on the Internet listing the values.
I was able to use a front sensor from a mark viii on my 91. The mark had a different connector than my 91, so I had the wire in both the female and male connector of the mark sensor to make it work.