Washington: Rev. Code Wash. (ARCW) § 82.08.0254; WAC 458-20-190

Rev. Code Wash. (ARCW) § 82.08.0254. Exemptions -- Nontaxable sales.
The tax levied by RCW 82.08.020 [Retail Sales Tax] shall not apply to sales which the state is prohibited from taxing under the Constitution of this state or the Constitution or laws of the United States.

WAC 458-20-190. (Sales to and by the United States -- Doing business on federal reservations -- Sales to foreign governments.)
(10) Sales to foreign governments or foreign diplomats. For specific details concerning the taxability of sales of goods and services to foreign missions and diplomats, contact the department's taxpayer services division at: 

Department of Revenue Taxpayer Services
P.O. Box 47478
Olympia, WA 98504-7478

or call the department's telephone information center at 1-800-647-7706 or visit the department's web site at http://dor.wa.gov.