I have two large circular planters (24 inches diameter) into which I transplanted a large number of tiny dactylorhiza seedlings over the years. As an experiment I have also sown a number of species (from seed exchanges, from friends, from my own plants and from the wild) directly onto the surface grit of these pots and now possess a couple of orchid 'mini-gardens'.
The initial compost was made up of the contents of a number of spent pots; some soil that the orchid seedlings were growing in and a tiny amount of garden soil.
Hope this helps?