In the Algarve region you can buy/exchange euros very easily at Cambios (Bureaux de Change or Change Shops), hotels and even shops. Many banks no longer offer foreign exchange, however. The best rates and commissions are normally found at the bureaus in larger towns. Worst rates by far are at the airports and hotels, so it’s better to avoid those, unless there is absolutely no alternative. The best thing is to keep an eye out whilst out shopping and always check the commission rates. You won't find much variation in the rates of exchange, as the change shops tend now to be owned by the same company.
The Cambios (Bureaux de Change or Change Shops) are open seven days a week in the main tourist areas. You may need your passport to exchange cash.
Keep in mind that there are always fees (hidden or otherwise) and Cambios don’t exchange money at the most favorable commercial (bank) rates. The best bet is to use an ATM/cashpoint to obtain euros from your home bank account, or use a non-fee credit card where possible.