Hello - I’m traveling to Italy, France, and Portugal during the summer and had a question about (i) the best place to buy international stamps (ii) if postboxes are available readily or if the only way to mail something is in the post office.
Let me know if I should be aware of anything specific for sending international postcards from these countries.
I recommend to post and ask those things of each countries Geographical sections 
at least Italian site there is a pinned thread about stamps 
I’m on my phone at the moment so I don’t remember how to add links
Edit: at least in Rome gotta be careful with stamps, they might try to sell you GPS tracked stamps that aren’t Italian Post’s own.
What I have understood about these GPS stamps are more expensive, they will take longer (if they ever arrived to destination) and they can be mailed only from the mailbox that are meant for them. So better avoid them.
Okay I managed to get one link here 
Hi,
In Portugal, the best place to buy stamps is at the post office (CTT or Correios). Most tourist shops ask if you want to buy stamps, but the answer is always NO. The stamps are sold at the prices they want (in Portugal the postage price is regularized) and the stamps may not always be valid (the stamps lose their validity after x years from the date of issue). Usually the stamps that are sold are ugly and self-adhesive (at post office), but if you ask, there are usually traditional gum stamps.
The current price is €0.95 for Europe (includes all European Union countries and regions, United Kingdom, Andorra, Russia, Belarus, Iceland, Bosnia and Herzegovina, Kosovo, Serbia, Macedonia, Switzerland, Turkey, Moldova, Ukraine , Montenegro and Norway) and €1.05 for the rest of the world and the United States (standard mail and up to 20 grams).
The best place to deliver mail is to the mailboxes next to post offices. They are usually identified, but if they are not (Red is for national and international regular mail and Blue is for urgent and international mail). In some branches there is a specific box for International Mail.
Thank you! Are post offices normally open during their posted hours?
Thank you and good tip! I will look through each geographical section!
Here you can also have useful tips of prices. For example in Italy you have to buy different stamps for Europe or to rest of the world.
I was in Italy in the beginning of May and bought stamps from a post office. I was staying in a small town and even there I got good service in English. There were red mail boxes every now and then, so it was easy to drop the cards.
I also wouldn’t buy those GPS stamps. The souvenir shops were selling those but if I had bought those I would had to go to the other town to mail them because you can’t use Italian post box for those and the nearest yellow box was in next town. 
@anikabrooklyn From june 21 italian tariffs for domestic and EU will increase of 10 cents, while for extra-EU they remain unchanged.
B (domestic) from 1,1 to 1,2€
B zona 1 (EU, Russia, mediterranean countries) from 1,15 to 1,25€.
It can interest you if you have to combine stamps to reach a tariff, in lack of the specific stamp.
For example two B will have same value of one B zona 2 (extra-EU except Oceania).