Just to say it, the reason copper or stainless work for a cathode is because a colloidal silver making electrolysis cell is basically an electroplating cell where metal ions only come off the anode (more positive electrode in the cell) where if it were a true plating cell, those metallic ions would plate onto the cathode. Its basically a one way deal.
Since no metal comes off the cathode, it can be a couple other acceptable metals as stated in a prior post, which as Kephra states, are much less expensive than silver and very easy to obtain (especially copper). As he says, if you want to use a silver cathode, go right ahead though its no better nor any worse than a copper or stainless one.
In our use of this "plating cell", its really "broken" in the sense that the electrolyte we use prevents the metallic silver ions we're pulling into solution from plating out on the cathode where, instead, the concentration of those ions in solution grows to the point we want.