the best substrate is Wheat straw but can also be grown on paddy straw and brassica straw but you dont get good yield on paddy and brassica straw. You can add cotton seeds as substrate , but you have to pasturize for about an hour in a custom made pasturise chamber. (King oyster, pleurotus sajor caju, pleurotus florida)
I have grown successfully oyster fungi on wheat straw and hardwood chips (poplar and eucalyptus). The moisture content, the particle size, pH and the pasteurization of the substrate are the main drivers. Good luck
I was just telling the alternative of Wheat straw , but if you are pasteurization the straw in the chamber then for 30 kg wheat straw you please add 150 gms of cotton seeds if available. If not then wheat straw itself is sufficient for cultivation of Pleurotus species.