If you want an used one:
I am spanish but not from Barcelona,im from madrid, but i can help you. There is a webpage called "www.segundamano.es" where people sell their used things. here is the links with the NOkia n900 results on barcelona.

http://www.segundamano.es/anuncios-b...1&fc=&sb=&st=a

You may use a translator...if you dont know so much spanish you will have a prob, cause you have to send a email to the seller.

If you want it NEW:

Simply, find a "The phone House" store and buy a brand new one for the ammount of 478 €
I think that they dont have stock...so probably you will have to go to all Phone House stores.

Hope i help, if you want more advice ,say it =)