You can replace this with a packet of instant yeast, if you have it. Unless you have a lot of patience, and in this case you can also prepare your own yeast with water, flour and a pinch of salt. You'll end up with a softer dough than regular bread dough. Let sit for 3 days, and the dough will expand into a prepared base for all the same breads and pastries you usually make with baker's yeast. Then add a bit of flour and let ferment for 2 days more.