I'm wondering how you know your milk has not come in. A lot of people describe horrible engorgement when milk comes in but this is not what is supposed to happen. If baby is feeding frequently and transferring milk adequately you may never get that bowling ball feeling of engorgement.
If baby is not having enough pee/poop then you may want to look at your milk as not having "come in" yet, but remember your baby doesn't need huge amounts at this time. It doesn't just suddenly convert from colostrum to regular milk, it is a gradual process. They arent completely different products, just different ingredients that change gradually over time to meet your babies needs.
Milk volume increase can be delayed by lots of things, retained placenta, c-section, gestational diabetes, thyroid problems, general hormone problems, contraceptive pill, PCOS can't think of more but sure there are some more.
I would get hold of my local hospital grade pump renter and start pumping. This will help your milk supply, and you can then top-up with breastmilk instead of formula. NCT usually has these types of pumps.