If your server is in an Active Directory domain, you can setup one of the servers as the Certification Authority for the domain, and use it to issue a root certificate (the certificate that will be used to issue all others certificates - for computers, users, web servers, ...).
More on that here:
Active Directory Certificate Services Step-by-Step Guide[
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Once you have your internal certification authority, you just have to use the IIS MMC to get a certificate for your web server that will be used for SSL encryption.