I knocked mine off the sideboard today and most of the medium fell out. I know that the stuff is not compost or garden soil but can't remember what it is called and if it can be purchased in small .quantities?
Looking at what appears to be needed, both in Mamya's link and on the RHS website, a potting mix designed for cacti and succulents should be ideal. It's sufficiently fast draining and any garden centre should stock it.