Formula is expensive

So I had planned on nursing my daughter until she was old enough to drink milk. My first child had formula from day one and it was so hard to afford so I was really hoping to stay away from it this time around. My little one is 4 months old now. Unfortunately I’m not producing enough milk for her so we’ve been having to top her up every feed with formula. But I’ve noticed we’re going through formula so much faster now and my supply is dropping drastically. My maternity leave pay is so low that I cannot afford to buy formula every week. I’m kinda panicking, what should I do? My baby needs to eat.