Where to buy/top up public transport tickets in Lisbon?

On a recent visit to Lisbon, I made considerable use of the public transport system. On first arriving at the airport, I purchased a "Viva Viagem" card, and loaded it with some funds. This worked pretty well, getting me bus, tram and metro rides, as well as some local train journeys outside of the city.

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However, the only places I saw and understood to top up this card/buy a new one were the ticket offices and ticket machines in metro stations. I did actually see one of these machines by the castle entrance as well. Other than that, I didn't see anywhere to get the tickets. As you can't pay on boarding for at least some trams etc, this left me in some difficulty at one point, being at a stop in Belém without sufficient credit. The metro website only mentions buying on the metro. The Carris website suggests other places, but I'm not entirely clear on where it was recommending, or how I would find them on looking around me near a stop (also, I hadn't read the page at the time).

So, where should I have gone from near the stop Pedrouços? How could I have known that at the time?

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