For simple stuff like that, I always start at Napa/Autozone/Online. The knock senor is a tuned crystal that sends out an AC signal when the block vibrates at the right frequency (that of knocking). Depending on what crystal and process they decided to use, it can be really cheap or really expensive.

91 YJ
2 Seater Sand Rail
Handfull of other fun toys...
And a brand new, spit shined, B.S.
Oh, and a job to boot.