Breastfeeding and formula

Stacie β€’ Mother to Ben πŸ‘¦, Bash πŸ‘¦, and due with Baby #3 March 2018, 420 enthusiast 🍁πŸ”₯πŸ’¨, lover of life 😍✌
If you know you're gonna be breastfeeding, should you invest in formula? 
I plan to breastfeed my son until at least 1 but my neighbor told me that I wouldn't be able to do it because it was extremely painful for her and it would be painful for me.
I'm gonna buy a couple things of formula, but I'm not gonna start stocking up on it until after I have my son and I know if I'm breastfeeding or not.