presuming you speak spanish, call a couple of banks and ask the rate to buy and to sell dollars to euro in us$ cash, and in tc(if you can get them free somewhere). also ask them if they accept $100 bills.

ask them if they have an internet site that posts the exchange rate.

the other places that may have a good exchange is the post office and the department store. you can call the department store and ask. do not know about the spanish post.

i spite of what others say, this is usually the least expensive way. just remember, you will probably save 2% over an atm machine. 2% of 1000 is 20 bucks. not worth much running around.

i do not know if spanish airports have bank cash windows and the rates they give.