Natrix natrix isn't CITES listed. Unless there's some european laws I don't know about, getting them imported into the US is entirely legal. Find a european breeder or willing to export them and have them shipped to you. Use a broker to make sure it's done correctly and you're set.

I've done this with a few species I wanted. It's no big deal.

Then the only answer to why there isn't more of them around is because... You didn't want to do the work to get them.

DT