Stamps may only be purchased in tobacconist shops or at the Post Office (Correos).
Most towns have a Post Office but do expect to have to queue when using Spanish postal services, which do not have a reputation for speed and efficiency.
You will often need to collect mail from the post office as larger envelopes and packages rarely leave the dispatch room. You will receive notification when there is something to be collected. It is commonplace for your mail to be delivered to a mailbox at the bottom of your street or to the President of your urbanisation. Do not always expect to open your mail before breakfast and always post your mail with plenty of time to spare.