Can I sterilise without a pressure cooker?

For grain, not really, because plain boiling does not reach the temperature needed to kill heat-resistant bacterial spores. You can pasteurise straw and some bulk substrates with hot water or lime instead of sterilising, which avoids a pressure cooker. But nutritious grain and supplemented sawdust almost always need 15 psi to succeed. A pressure cooker is the one tool worth buying early.