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There are many old book shops in Lisbon here are a few tips:
If you're looking for shops with second hand books you will want to look for "alfarrabista". That't a specific word that designates old book shops or old book sellers in portuguese. Since you are looking in Lisbon, try: "Alfarrabista lisboa".
List of bookshops
In a quick search I found this list of old book shops in Lisbon (it's in portuguese, but it's easy to spot the address.). A map of bookstores an libraries can be found at LibraryThing local.
Markets
There are a few second hand markets and flea markets where you find stand with old books. Try "feira da ladra". That's a good place to find many second hand stuff including books. There used to be a few second hand stands in "praca do comercio" also. I don't know the situation now.
Shops
There is a very well known street with a some second hand book shops it's called "rua do alecrim". It's around the city center and it's a nice walk (down) from Camoes to Cais do Sodre.