T-Mobile sells a “Prepaid 3-in-1 SIM Starter Kit” online. It’s listed at $15 but there’s a promo code to purchase for 99 cents. https://prepaid-phones.t-mobile.com/prepaid-phone/t-mobile-prepaid-3-in-1-sim-starter-kit
They’re also available to pick up in-store, but in addition to the wait time, you’d pay $10 for the SIM and $10 for the E-PIN (plus tax). (But also get a $7 starting balance.)