You could buy an UK keyboard, which unlike the US one very good and intuitive to write national characters.
or
You can buy an UK or any other ISO layout and then use the stickers from 4keyboards, latkey to mate it dual language or just Spanish, depending on the stickers used.
or you can look at the classified section for group buy of the Spanish is keycaps.
I'm in italy, and I have the same problem. BTW I went for UK layout after I've discovered how handy is the UK layout.
(really not the layout per se, but how works the the
UK extended logical layout, in windows linux and so on)