Not possible. T999L was released with 4.1.2, 4.1.1 doesn't exist.
If you flash 4.1.1 for another variant, you will brick.
If you are on a regular plan, pay the bill for 2 months and then T-Mobile customer service will unlock it for you. Otherwise buy an unlock code online.
Sent from my SGH-T999 using XDA Premium 4 mobile app