Jeff,
at the same office where they are issued. Normally the designated National Police Station where they also issue Spanish Passports and IDs
I am pretty certain you have to get a form from that office, to make a payment at any bank, and then complete an application form.
I cannot remember the last time I need to produce the original cert, as 'most places' where you may need to provide the number, can check details on-line. Example, Making a denuncia at a police station, with the NIE they can call up all your details.