T-Mobile is now accepting orders (as opposed to pre-orders) for the Nexus 6 through the T-Mobile website in both 32 and 64GB trims, though the only color option remains Midnight Blue, as it will likely always be for carrier-bought versions of the device.
Selecting either storage variant allows you to choose standard 2-4 day shipping or upgrade to overnight service for $12, though nowhere in the checkout process is it actually made clear when your Nexus 6 will ship, only the amount of time it will take to arrive once it does. Given the scarcity of Nexus 6s at T-Mobile retail locations today, I wouldn't be surprised if orders from the website took some extra time to arrive.
Still, you can give it a try either way - financed, the 32GB model will cost you $27.08 a month, and the 64GB just a couple bucks more at $29.16.