For purchasing SSL Certificates please visit: http://www.exchangecertificates.com
Unlike the latest version of Exchange, 2007 does not have a certificate wizard in the GUI, all the SSL Certificate requests need to be performed and applied via the Microsoft Exchange Shell.
If you are not used to the command line this can be very confusing, even if you are used to the command line this can be fraught with errors in getting the SAN/UCC names correct, adding specific names etc. Here there is a handy FREE utility that will do all the hard work for you: http://www.u-btech.com/products/certificate-manager-for-exchange-2007.html
The tool will generate SSL requests, create self signed certificates and allow you to fully manage your existing certificates.