Here are the materials in Bonnet v. Ute Indian Tribe:
The issue before us is whether a subpoena duces tecum served on a non-party Tribe and seeking documents relevant to a civil suit in federal court is itself a “suit” against the Tribe triggering tribal sovereign immunity. Exercising jurisdiction under 28 U.S.C. § 1291, pursuant to the collateral order doctrine, we hold the answer is yes. We therefore reverse the district court’s denial of the Tribe’s motion to quash based on tribal immunity.
And the briefs:
Lower court materials here.