I don’t know of any sites dedicated to the spies. However, the town of Estoril, very close to Cascais was supposedly the epicenter of the spies, etc. A bit related - the Estoril Casino was frequented by Ian Fleming. It was there he came up with the idea of James Bond. There are stories of an old hotel being torn down and the walls being filled with miles and miles of wires for bugging the rooms. The Palacio Hotel in Estoril is still there and was also likely frequented by some of the spies, etc. It’s a lovely old hotel and worth a visit for a drink. I know there are some private guides in Lisbon who will do tours about the spies in WWII, but I don’t have their names - sorry.
And, lastly, even though Portugal was thought to be neutral, Salazar ( dictator at that time) played both sides in the war for his monetary gain. I don’t know much about that story but think it’s quite fascinating.