Thanks for the question. The following purchase page may have the information you are looking for:
If you buy a SIM-free iPhone, you will need to purchase a GSM-compatible nano-SIM card separately. In the United States, you can purchase such a SIM from AT&T or T-Mobile. When you travel internationally, you can use a nano-SIM card for iPhone 5c from a local GSM carrier. The unlocked iPhone 5c model is A1532 (GSM).