You can buy stamps to send letters in any tobacconist's ("estanco"), in some hotels or in post offices. They can then be posted in any of the yellow postboxes spread around the city.

Central Post Office.
Vía Laietana, 1. Tel. +34 93 310 04 04.
Mon-Sat.: 8 to 20 h. Sun.: closed. Stamps, parcels, telegrams, telephone, telex and fax.

Eixample: Aragó, 282 (Pg. de Gràcia). Tel. +34 93 216 04 53.
Mon. to Fri.: 8 to 21 h. Sat.: 9 to 14 h. Sun.: closed Postal service only.