Where can i get a iphone 5s sim card?

Ok so I have an iphone 5s and it didn't come with the sim card/memory card and I called the company and they didn't have it the company was boost mobile do any of you guys know where can I get a memory card/sim card

Ebay

Bad news Boost, Verison, and Sprint use CDMA phones (No Sim card). AT&T and T-Mobile use GSM (Sim card) phones.