What Type of SSD is Used in the 2013 Mac Pro?
These particular Mac Pros use an Apple designed PCIe 2.0 x4 interface. If you're wanting to replace the internal drives your options will be limited.  

I would check out the following instructions provided by Apple to determine your happy to proceed. If so, I suggest you purchase your SSD from a reputable retailer. For example, the following are some well-regarded sites:


Mac Sales 
Simply Mac
RAM City
Macfixit


By buying from a reputable store you'll be sure to get an SSD fit for purpose, regardless of manufacturer. Being reputable means they're only going to sell something that works and they'll stand behind the product they sell, regardless of the brand. I would also check out Amazon as well for particular models once you know what you're after, as you'd probably find you can buy cheaper from there.

Another option

If I was you I would seriously consider keeping your current drive and using an external Thunderbolt enclosure to increase your storage capacity. If speed isn't so important, then you could use external USB drives instead.