In the most major communities in Iceland you will find Post offices.
The offices are normally open in the Reykjavik area 09:00 – 18:00 Monday – Friday, but post offices in smaller communities may open between 10:00-12:00 and close at 16:00.
It is also possible to buy stamps by many hotels, grocery stores, bookstores or tourist centres. Call 580-1200 or see http://www.postur.is for more information.