Try Amazon.com or ebay. I have been to Portugal. A very lot of Portuguese people there, 45 years old or less, speak English. You shouldn't have much of a problem but it never hurts to learn some of there language.
If you want to buy it from Portugal then it should not be a problem to find one. You can buy it from a city stationery shop or a book shop. They call it Papelaria / Livraria.