In store only. Metro by T-Mobile has it for $100 when you switch + activation fee.
Walmart has it for $199 on installment plans with Verizon ($8.29/mo. for 24 months) or AT&T ($6.63/mo. for 30 months). The 128GB model is $249.
Now features the A13 Bionic chip, Touch ID, 12MP camera and wireless charging. There's also no headphone jack.