Do you know when they were born? Spain has a civil registration process for births that has been in effect since 1871. The records are compiled in a national database at the Ministry of Justice. You can ask for copies of their birth certificates from the Ministerio de Justicia. Here is more information: http://www.mjusticia.gob.es/cs/Satellite/Portal/1292344469707?blobheader=application%2Fpdf&blobheadername1=Content-Disposition&blobheadervalue1=attachment%3B+filename%3DTo_request_CERTIFICATES_of%3A_Birth_Marriage_Death.PDF
One always starts with the most recent birth that they are sure about and traces backward. There is no way to start in the late 1200s and trace to 1930. Therefore, the place to start is with the births of your late grandparents. Once you have those you can request the birth certificates for their parents, etc. back to the earliest civil records.
The majority of Jewish records have never been transcribed and translated and made available online. Some family descendants have researched records in person and graciously shared what they have learned on various web sites. However, only the tiniest fraction of what exists is available without research in person. That would be the final step once you have exhausted all the civil records.