Syndicate Supplies is a store available to every Agent where you can buy special and unique items, or ammunition. Such shops don't accept regular money (NIC) but only Syndicate tokens issued by the megacorporations. You can obtain these tokens by completing assignments.
The price is displayed below every item, but you have to keep your tokens in your private storage to be able to pay with them.
The selection varies in every Supply shop, for instance if you're looking for unique missiles, you might want to check Syndicate Supplies shop on Truhold-Markson zones' terminals and outposts.