It's a real bummer that the Nexus 6 has been so hard to find, but supply is starting to catch up to demand. T-Mobile has enough in stock to be offering a nice little Cyber Monday deal. The Nexus 6 is $50 off if you buy it today.
T-Mobile doesn't apply any weird branding to the device like AT&T does, and it's SIM unlocked. The carrier only offers the blue Nexus 6, though. If you want the cloud white model, look elsewhere. The final price if you pay up front is $601.99, down from $649.99. You can also finance it for $25.08 per month for 24 months. The 64GB model is $651.99 or $27.17 per month financed.
Update: Apparently T-Mobile's idea of "Buy Now" is "we'll send it in a few weeks." Bummer mk2.