What is an alternative to leaf mould?

I have bought 5 ferns and it says plant in 1/3 sharp sand, soil and leaf mould. I dont have any mould so where do you buy it or what can I use instead?

 

I use composted bark instead of leaf mould when I run out. Cheapest source is a Non-peat growing bag. Some of the stuff is a bit 'rough' so you could sift it, but the ferns will not care too much.