Most of the big suppliers are in Portugal, e.g Vodafone, 3, Optimus. Not sure about Orange.
I use Vodafone for a local mobile, as their network coverage seems to be really good. I just bought a Vodafone sim at the airport and use that in an old phone,. But they are also available at the large shopping centres.
I top it up using online banking from my Portuguese account. You can top them up in the Vodafone shops and also via cash machines (called multibanco in Portugal).
That is my least favourite method, as I can never understand the instructions.

I also bought a vodafone pay as you go dongle for the internet at the airport. From memory, this was about 50 Euros and it had 20 Euros of credit on it. Then I reload it with 10 Euros of credit each time I visit and it lasts about 2 weeks. This is really a good deal, as I have never come anywhere near using my credit.