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Home Depot. It cost about $1.50 or so for a 1 ft square tile.

I actually have a shapton lapping plate that I used once to lap my 15k shapton. For the norton I prefer wet/dry sandpaper on artificial marble.

Some nortons are flat from the factory, some aren't. I thrashed around for a month with mine before I broke down and lapped it, then popped out a shave-ready razor first try :cen . Better safe than...