You can either buy an SSL (X.509) certificate or generate your own (a self-signed certificate) for testing or, depending on the application, even in a production environment.
Good news: If you self-sign your certificates you may save a ton of money.
The term 'SSL certificate' has persisted, and will likely persist for the foreseable future, because given the choice of saying 'SSL' or 'X.509' the former tends to roll off the tongue more comfortably.
Doubtless a linguistic expert could wax lyrical over the S sound versus the X sound.
Currently published RFCs are pointed to https:// XXXX which contains various information and links to the text (normative) reference and a PDF (non-normative) version. The major use of SSL (X.509) certificates is in conjunction with the TLS/SSL protocol.
What are colloquially known as SSL certificates should be referred to as X.509 certificates.