Where can I buy a replacement Mac pro 2013 SSD drive?
The MacPro uses a proprietary interface - at least when it was first designed - for their SSD's It is a PCIe 2.0 x4 spec.  

It was called "proprietary" because no other "workstation class" computer used these types of drives.  They typically went with SATA or SAS (enterprise class of course)

The best price you are going to find for these drives is through Amazon. You may be able to win an auction on ebay, but for the time and hassle, I usually just went with Amazon.

These drive only work with the "grey" cylinder MacPro.  You can get more info from everymac.com

What you might want to consider at this point is sticking with the SSD that you have and adding a thunderbolt drive.  I personally use the WD Thunderbolt Duo 6GB with my iMac (iMac has 256GB SSD).  It is super fast and expandable.  It has user serviceable drives so you can upgrade anytime you want.  There are other options out there, but after all my research, I picked this one and I have been extremely happy with it. 

Update (3.31.16)

A year later and this hasn't changed much.  OWC is apparently the only aftermarket manufacturer of these SSD with the PCIe 2.0 x4 interface.

As far as SSD options, they seem to stop at 2013 for the MBA. I know, I have a relative that spilled milk on an MBA and have been looking for a USB to 2014 MBA SSD enclosure to transfer data and I can find none under $100.

It seems that the 2015 MBA SSD will work in the Mac Pro as they use the same SSD model (see below).  Source:  EveryMac.com





The 2014 MBA uses the PCIe 2.0 x2 so I cannot say if it will work or not (and I would be unwilling to test out this theory given the high cost of the components).  Source:  EveryMac.com