Buying a locked iPhone?

I was planning on buying iPhone 5 or 6. The problem is that I already have a contract with Cricket through my current phone and I want to use the same sim card in the iPhone I'll buy. Cricket does not have the iPhone I want and I wanted to buy one from Best buy or Fry's electronics. My budget is $200 and I don't want to pay extra money to unlock the phone. Do I have to buy an unlocked iPhone or a locked iPhone?

Buy unlocked, but I think the apple stuff self locks to the first carrier's sim card it sees