For what purpose would this certificate be? There are code-signing
certs, web access certs, e-mail certs, and others. Often, one type of
cert can't be used for other purposes.
In general, you purchase a cert from a well-known Certification
Authority, like VeriSign or Thawte. Thawte also offers a free email
signing cert for personal use. See
http://www.thawte.com/secure-email/personal-email-certificates/