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

Considering using a European formula?

FDA site including information on using imported formula

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

Letter from UNMC dietician with information on alternatives to formula products (includes information on  elemental formulas):

UNMC Dietician Parent Letter/Resources

Additional information and resources from Healthy

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 regarding Donor Breast milk

The Academy of Breastfeeding Medicine provides information about re-lactating and offers additional considerations about use of donor breast milk.

Re-lactating is the process by which a person who has given birth may ramp-up milk production after a period of not lactating or partially lactating.

Additional recommendations from the Academy Breastfeeding Medicine