Waking up at 5:30 AM to a text message saying your opening infant teacher is sick is the absolute worst way to start a day. Scrambling to find a substitute at the last minute usually ends with the director stuck in a rocking chair all morning.
You cannot wait until you have a sick call to look for a sub. You need to build a warm pipeline all year round.
Start by recruiting locally. Reach out to early childhood education programs at nearby community colleges. Those students need classroom hours and are highly motivated. Next, look at your recently retired teachers. Many of them miss the children but just do not want the grind of a forty-hour week. Finally, treat your substitutes like absolute gold. When a sub walks in, have their schedule printed, introduce them to the team, and make sure they get their scheduled break. If you make it easy and pleasant to work at your center, they will always answer your text first.