Where do I buy a new sim card for my Sprint iphone 4s?

I went into Sprint and asked them how I could get a new SIM card since my husband, who is in Afghanistan, misplaced his. They looked at me very confused and said they could not give me one, or let me buy one unless I had one to swap out [which I obviously do not]. They told me to contact Apple, but I got to thinking about it, and now Im confused. Isnt the SIM card something that comes from the phone carrier? I have no clue where to go from here.