We are not sure if the EORI must be obtain from the country the supplier is based in you are dropshipping for.
However, we do know it is possible to download a form from the Customs website in Spain, specifically for those people in need of a Spain's EORI number who are not established in the EU,Link hidden: Login to view
However, the website mentions you would need to present this form in person in any of Spain Customs Offices, or have a representative do this for you; maybe this is why Spain Customs informed you the cost was 1000 EUR: their fee for the service.
Another useful page in Spain's customs website (please use google translate),Link hidden: Login to view
We hope we were at least of some help. We are not experts, so please do make sure all is correct before proceeding with any paperwork or negotiations.