VPN and certificate

We are working to develop computer access for our company, our users sits in different locations on the glob and we need permanent access to them not an instance access, the goal is to take remote access with no user interfere. In order to achieve secure connection we are planning to use VPN and certificate ( when we require to access computer over the internet).
Since it is not reasonable  to purchase certificate for all our computers (more then 500) we are searching solution that could manage the certificate and allocate it per connection, when connection close, certificate will be back to the certificate pool.
Appreciate any suggestion regarding how to establish this.
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David Johnson, CD, MVPOwnerCommented:
create and use your own certificate authority.
There are several articles on this subject
http://blogs.technet.com/b/yungchou/archive/2013/10/21/enterprise-pki-with-windows-server-2012-r2-active-directory-certificate-services-part-1-of-2.aspx is one of many

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