There is a nice howto available, which explains how to install a SSL-certificate within ISPConfig for a website. http://www.howtoforge.com/faq/14_49_en.html I took all the steps with ISPConfig 2.2.15 and everything works perfectly, except one thing at the end. When the official SSL-certificate has been saved within the SSL-field, ISPConfig does not restart Apache2. Restarting Apache2 is necesarry, otherwise the certicate is not trusted when you visit the https:// website. For this reason i suggest to add the line: "Restart Apache2 afterwards with: etc/init.d/apache2 restart". It would be more nice if ISPConfig restarts Apache2 after saving the SSL-certificate automaticly.
@Edge, I bought it at www dot sslcertificaten dot nl It is a "Comodo InstantSSL: Certificaat met bedrijfsgegevens". Works great! Fast delivery and great support!
Apache should be restarted automatically, however I've once seen this behaviour that it doesn't. An ISPConfig restart helped in that case (not sure what was the cause...).