If you are in a spanish speaking country and you need to buy postage stamp to send a letter or a postcard you need to use this sentence:

-Necesito sellos para el Reino Unido
(I need postage stamp to the UK)
or
Cuantos sellos hacen falta para enviar esta carta al Reino Unido?
(How many stamps do I need to send this letter to the UK)

Some more vocabulary that might be useful at the Post Office is:
-Oficina de Correos (Post Office)
-Sobre (Envelope)
-Postal (Postcard)
-Correo Urgente (Express Mail)
-Paquete (Parcel)