The only wireless phone "store" I know of at McCarran is pre-security in T1, upstairs on that big "bridge" between the C/D gate security and the check-in area. It's right next to the walkway that goes to the PreCheck/elite/F line for D gates. But they're an independent shop IIRC - and quite small, even calling it a kiosk might be generous - so I don't know if they sell the T-Mobile prepaid sim cards.

Fashion Show Mall (just north of Spring Mountain on the Strip) has a ton of wireless kiosks; you could check if there is a T-Mobile store as well.

You could even try 7-11 as they sell cheap phones with T-Mobile sims in them, and then you can just toss the phone and keep the card. There is a 7-11 at Hawaiian Marketplace just south of Harmon on the Strip. Call first to see what they've got before making your way over, though.