IPhones that can read already activated T-Mobile SIM cards?

I asked my dad if I could get an iPhone if I could find one somewhere cheap enough that I could afford it. He said only if I can find one that can read regular T-Mobile SIM card, and is already unlocked to where it will work on the T-Mobile network.
Does anyone know where, if they even exist, they can be bought/ordered?

Factory unlocked or att iphone that have been unlocked are the only iphone tha will work on tmobile, and no 4g only edge 2g.

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