The Spanish registry is still pretty closed and without a legally registered company in Spain, it isn't possible to register a .ES domain :
Assignment of a regular domain name will be to Spanish or foreign natural persons with legal residency in Spain and organisations with their own legal personality constituted according to Spanish Law, registered with the corresponding Public Spanish Register (Registro Público Español)
Full rules on .ES registration are available at https*://www.nic.es/ingles/rules.html.
If you're keen on a .ES though, there are a few companies who will register one for you using a native Spanish company as the registrant. Not an ideal solution as it's both expensive and takes control of the domain record away from you, but when needs must...
R.