There are many sources on the web- google 'rare earth magnets'.
Here is a link to just one possible source: http://www.rare-earth-magnets.com/SearchResult.aspx?CategoryID=21&KeyWords=Disc&All=True
I have ordered from Indigo.com and been pleased.
For a carnifex, the 0.25" (6.4mm) should be plenty strong.
I have tried using a magnet on one side, and a nail-head on the other. That is simple.
Magnets on both sides gives a much strong hold, but requires care to align the poles correctly.