There are several commercial CAs available. The mechanism for signing a certificate using a commercial CA depends on which CA you choose.
An advantage of commercial CAs is that they are often trusted by a large number of people. If your applications are designed to be available to systems external to your organization, use a commercial CA to sign your certificates. If your applications are for use within an internal network, a private CA might be appropriate.
Before choosing a commercial CA, consider the following criteria:
What are the certificate-signing policies of the commercial CAs?
Are your applications designed to be available on an internal network only?
What are the potential costs of setting up a private CA compared to the costs of subscribing to a commercial CA?