The US Postal Service currently provides employees with a voucher they can use to purchase uniforms from any authorized vendor. That may change soon, based on the following solicitation posted on the FedBizOpps web site:

To obtain sources for market research to support the U.S. Postal Service’s National Uniform Program. Under this effort, the successful Supplier shall be responsible for full program management of the Uniform Program (e.g. certifications, marketing, distribution, delivery) including manufacturing and providing all uniform items (e.g. dress uniforms, work clothes, accessories, foot wear, outer wear etc.).

via To consolidate the Uniform Allowance Program under one umbrella for Cost Management. – 2W-11-A-0018 – Federal Business Opportunities: Opportunities.