I got mine from this link that megatronium provided above...
https://twitter.com/jesse4685
You can check the availability of the hookup code by typing it in on this page and looking at the "invites available" field.
https://promotions.t-mobile.com/TheHookupLookup
In fact I just checked and I9I8R5J4 has 1 spot, W9N1S4N7 has 5 spots, and A4A4E2C2 has 5 spots. There are likely several others on his twitter that have spots too. (Note: the 5 spot ones may not be fully activated yet in their system, so you may have to wait a bit before it will accept them. The 1 spot one is clearly currently active, so it will work assuming the spot is still available.)
When you find a working code, enter it at
https://promotions.t-mobile.com/TheTMobileHookup After you enter it and it is accepted, you have 14 days to signup for service (either online or at a T-Mobile store) under the number you provided on the form. Even if you are unsure if you are going to transfer service to T-Mobile, it's best to at least submit the form with a hookup code to reserve the promotion in case you do.