Understand that supplementing with anything other than pumped milk will decrease your supply.
Try gripe water. Sucking on fists is often a sign of upset tummy.
As much sucking as can be done at the breast, should be done at the breast. It will increase your supply. Some babies, like my first, have a need to suck to soothe that far exceeds their hunger. Consider trying a pacifier, her thumb, or your finger (nail to tongue, pad to palate).
Oatmeal is considered great for supply.
How long does she usually spend on a side?