Subpoena
From LegalLanding
Subpoenas are allowed by FRCP Rule 45.
Discovery from nonparties is more limited than discovery from parties. A subpoena is necessary for depositions and document inspection or inspection of premises, and that is all. A party must avoid unfairly burdening nonparties and alert them in the subpoena of the right to seek protective orders. The court may require reimbursement for costs of responding.
As an officer of the court, a lawyer has the power to issue a subpoena on his own. If this power is abused, the court will come down hard. Subpoenas must be served personally. There is a field in the subpoena where you can describe information you want that person to produce, leading to a subpoena duces tecum (latin for, essentially, bring your documents).
If you subpoena a nonparty expert, then you have to pay for their time during a deposition; otherwise they don’t get paid. You also have to pay for copying documents that you subpoena.