I don't believe that they are individually replaceable. I think you will have to order a complete replacement tremolo.
See this site, which says:
The knife edges of the trem become worn or damaged and require filing to get back to a proper knife edge. This is also rarely seen on
Edge variants [except Edge3!] and some "mating" of the parts is
actually preferred as they form to each other, and their specific use.
Cheaper trems are more prone to wear. The same procedure as fixing a
fat/flat knife edge, see this section to walk you through shaping the
knives. Note, you do NOT want the knives sharp. You cannot replace
knife edges if you screw them up or they're shot, Ibanez refuses to
sell them, or the trem baseplates alone, you need to replace the
bridge at that point.
Here is the knife edge-sharpening page mentioned.