something like a piece of slate is ideal imo.
actually anything flat is good. ada makes riccia stones which are pretty flat but i wouldn't spend the money if i didn't have to.
like stated above, riccia will not attach to anything.
you just tie it down and wait for it to grow. after a while, you'll have to re-tie it again because it'll get to big. i'd had lots of success using bath poofs. its easy to use and the riccia will grow out nicely. this is what i'm talking about. fairly cheap, $1 at walmart.