You can't download NSX for vShield, I specifically denied your request...
Actually, this is another example of VMware's re branding and use of language that was bound to confuse users.. I was also confused and found this post while searching..
There is not specific product called NSX for vShield. There is however, a license called NSX for vShield that is embedded in the NSX manager. As long as you have NSX manager installed this license is available for free, but to leverage it you still need to appropriate vSphere license, which is anything from vSphere Essentials plus and upward.
Just download NSX for vSphere and deploy NSX manager according to the documentation.
Judging by some of the responses, although there isn't a post marked as correct, I'm thinking you already figured this out..
Now, IF I'm wrong, then correct me.