So maybe the Galaxy S6 doesn't strike your fancy because of the non-removable battery or lack of a microSD card slot. No problem—you can pick up an unlocked Galaxy Note 4 right now on eBay for a relative pittance of $470.

2015-04-07 13_41_04-Samsung Galaxy Note 4 4G LTE GSM N910A Latest Model Factory Unlocked 32GB FRB _

This is the N910A model, meaning it's intended for use on AT&T. It will work on any GSM carrier, and it should get LTE on T-Mobile via band 4. HSPA+ on T-Mobile will only work in areas that have the 1900MHz re-farm active. You should have no problem on AT&T and its MVNOs. Also be aware this is a refurbished device.

The price includes free shipping in the US, but it will ship worldwide if you want to pay for it. Sales tax will also be charged in California.