You can buy pieces of cork or slabs of wood on eBay. Andy's sells both rafts and sticks for mounting. When I need a stick I go into my yard and find a suitable one that's fallen from a tree (I like magnolia and oak).
Make sure you boil them before use to get rid of any nasties which may lurk in the wood.
Any particular nasties? Only I would prefer if I could keep the lichen alive.