The Samsung Galaxy S III LTE is available from T-Mobile's web site and right now only the Marble White model can be purchased. The nation's fourth largest carrier is offering the phone for a down payment of $69.99. The rest of the money is due in 24 monthly installments of $20 for a total cost of $549.99. This is the 16GB version of the phone and it is unknown if other variants are going to be offered. For just $80 more, T-Mobile customers can buy the newer Samsung Galaxy S4.
If you are not anywhere near T-Mobile's LTE coverage, you can still purchase the older model without LTE connectivity for the same $549.99 price. This model still has 4G data speed through the carrier's HSPA+ pipeline.
source: T-Mobile via AndroidCentral