I personally loathe most commercially offered Slates in that they all look like a magician's prop now days... let me clarify...
The days in which slates of that type were common place are more than a century behind us e.g. they do not make any sense UNLESS you have some kind of haunted tale, such as T.A.'s School Boy story in Mind, Myth & Magic that lends validity and reason for exploiting something so archaic.
When I did the Marilynn Monroe and John Belushi Seances in Hollywood long ago I used mirrors... for Marilynn the message showed up in lipstick for John...well, a line of cocain of course...

The thing is, mirrors are far more typical in today's world than writing slates too, there are mirrors of various sizes already set on a compact styled arrangement which lends to you a significant amount of advantage.
Picture Frames can give you the same kind of freedom and message bearing advantages, something Mary Tolmich (sp) revealed to us a few years back. But we can go significantly further with her basic idea given the new static plastics and so forth that are on the market.
BUT if you still want to do something with a slate I would suggest you simply go to a construction site and find yourself an old piece of slate roofing single or two, then contact the good folks at Daytona Magic and tell them you want to order a couple of sticks of GHOST CHALK... this is the only method I use for slates now days, if and when I use real slate. You will find something similar mentioned in Corinda in which a sceptic pencil is used such as those that used to be used for cuts.