If you buy an iPhone through the US Apple Online Store, one of the options in the process after clicking the Buy button is to select one of four listed carriers for your phone plan. Apple also notes: Connect to a carrier through us and your iPhone will arrive ready to go. All iPhone models are unlocked except those purchased with AT&T Installment Plans.
You will need to do some homework first to see if any of these plans meet your requirements.
This offer does not apply in my online Apple Store in Australia. It may or may not be offered in other national stories.
If you opt not to include one of these carriers in your purchase you will need to select another carrier of your choice and get a SIM from them once you have received the phone.
You will need to ask at your local Apple store if this applies for a purchase made at the store.