I saw this product on macworld today. It's a 256GB or 512GB external SSD that runs really fast due to the fact that it's 2 smaller SSDs running together in a RAID over thunderbolt. They advertise 700MB/s read and 400MB/s write. Starts at $300 for the 256GB.
Contact your favorite Apple Service Provider and ask for a 1TB replacement SSD for your nMP.
I imagine they exist for warranty replacement in case you are unlucky enough to have a failure in your nMP.
I also imagine it might cost $$$ + an arm and a leg
As mentioned in the other thread, LaCie has the Little Big Disk v2 which has 2 x 512GB PCIe SSD inside. This has been shipping for a few weeks and is purported to be the fastest/smallest Thunderbolt2 storage device with up to 1375 MB/sec thruput. Out of stock now according to their web store.
Slightly slower and shipping for a few years is the Thunderbolt 1 version of the Little Big Disk SSD which delivers just over 600MB/sec when configured as RAID-0; there are 2 SSD versions available: 512GB and 1TB. The latter is the version with 2 x 512GB SATA SSDs inside. So happens that a bunch of 2x256GB=512GB SSD refurbs are available here: http://www.lacie.com/us/products/clearance/products/?id=10007
There are also a few Rugged Thunderbolt SSDs available that also have USB 3.0 connections so the drive can be used with computers without Thunderbolt ports.
Though the nMP uses similar technology, in the Little Big Disk v2, the PCIe SSDs used in LaCie's standard M.2/NGFF edge connector pinout is different from Apple's proprietary edge connector. So only an Apple or yet-to-be-designed 3rd party expansion SSD would be required.