How to get a SIM card for an iPhone 5S?

My dad bought an iPhone 5s from a guy and he didn't ask if it came with a sim card and what company it was because he doesn't know anything about smartphones. I went to the store and they don't sell any and I checked on eBay but I'm pretty sure i need to know what company the phone is. So does anyone know how I can get a SIM card or will I need to tell my dad to ask the guy?

First of all you need to find out if the phone is network locked or not. Tell your dad to ask the guy he bought it from. If its unlocked, get a contract with a cellular carrier. If its locked ask the carrier if they can unlock it. If they say no its probably not a good sign (the phone could be stolen or not paid for). Then you would have to use an unlock service. Many sites offer those but unlockcode247 seems the most trustworth (i used their services once, quick and reliable. )
Hope this helps

Buy it

The guy would be the easiest way to find out which carrier it is locked to, if it is locked to any at all. Of course any carrier store would have SIM cards (well GSM carriers anyway)