Are you having difficulty finding formula locally? These sites may help find formula retailers:

https://www.similac.com/products.html

https://www.enfamil.com/store-locator/

https://www.storebrandformula.com/buy-baby-formula.aspx

Not sure what formula you can substitute? These documents provided by the Nebraska Women,  Infants and Children program provide recommended alternatives to formula products: 

Standard formulas 

Gentle/Comfort formulas

Sensitive Formulas

Formulas for Spit up/reflux

Additional information and resources from Healthy Children.org

What about donor breast milk? From The American Academy of Pediatrics:

Pasteurized donor milk may be considered in situations in which the supply of maternal milk is insufficient. The use of pasteurized donor milk is safe when appropriate measures are used to screen donors and collect, store, and pasteurize the milk and then distribute it through established human milk banks. The use of nonpasteurized donor milk and other forms of direct, Internet-based, or informal human milk sharing does not involve this level of safety and is not recommended. There are risks of bacterial or viral contamination of nonpasteurized human milk and possible exposure to medications, drugs, or herbs in human milk. Currently, the use of pasteurized donor milk is limited by its availability and affordability.

FDA regarding Donor Breast milk 

HealthyChildren.org regarding Donor Breast milk