Picked these up at Lowes for $9.99 for a sheet of 144 plugs and comes out to just under 7 cents for each plug. They are marble which makes them calcium based and the frags are encrusting over them nicely with no ill effects and being marble they are heavy enough that they stay on the frag rack even with 2 Tunzes blowing on them, if a problem staying on your rack you could buy acrylic rod and cut in short sections and glue on the bottom of the tile and you have a pretty cheap frag plug.
Works well for me and thought I would pass along.