SSL certificates are purchased by your company. There are several companies that can provide them. The first point of contact is probably the company where your domain is registered. Major providers of SSL certificates include:

• GoDaddy (www.godaddy.com)
• Thawte (www.thawte.com)
• Symantec (www.symantec.com)
• DigiCert (www.digicert.com)
• Comodo (www.comodo.com)

There are other providers as well that can be found via a web search.

Note that there is a cost for an SSL certificate and it must be renewed yearly.