There's only one add-on type table saw riving knife that I know of and it was built for newer versions of the Delta table saws like the "Contractors" series, but it will fit some of the other table saws of similar design. Do a Google search for " B.O.R.K ripping knife" to find his website, a youtube video, and other information about his product. You must search for that word exactly with the periods between each letter to find his site.
I have not ever seen or used one of his riving (ripping) knives, so all I know about it is what you will see on his website or the youtube video.
Central North Carolina