Q: Where to buy ssd 512 Gb for a Mac pro late 2013?

An Italian electronics store?

Or try MacSales.com  They ship world wide.

Try eBay and similar. Make sure you get one for a Mac Pro. There are ones available that are for a MacBook Pro but work I the Mac Pro but are slower than the ones for the Mac Pro. The SSDs for the Mac Pro have a black heatsink while the MacBook Pro ones do not have a heatsink.

OWC/MacSales sell SSD but they are slower than Apple's SSDs.

Apple does not sell their SSDs to the public.

PCIe-SSD for New Mac Pro 2013

http://barefeats.com/hard197.html

Care to tell from whom you purchased it?

Have you done a sped test and compared with the one that came with the Mac?

Some SSDs made for a MacBook work on a Mac Pro but they are significantly slower and do not have the heatsink

Black Magic is available free from the Mac App Store,

If you report results make sure you says what tests you did.

The following compares OWCs SSD with Apple's

http://barefeats.com/hard197.html

No, I do not know of any others

You can only use a county's Store if you are located in the country (IP address and other info is made to check that) and your ID payment method is associated with that country.