I haven’t been to Ljubljana but have visited other places in Slovenia and most post offices (Posta Slovenije), carry a selection of nice view cards and special stamps. Some even sell a pack of 10 cards for 3,50€.
International postage is currently 1,50€ (or a C-rate stamp), but you can also mix denominated and non-denominated stamps (A, B, and C) as long as you reach 1,50€.
Here are some of the latest stamps:
https://www.posta.si/zasebno/znamke-in-filatelija/galerija/postna-znamka#k=StampYear%3A2023#s=10
As for Croatia, I’ll link my reply from another topic here (there are also more replies regarding stamps there):
Almost all post offices in areas with lots of tourists sell a large selection of postcards and cards are generally widely available in souvenir shops and kiosks, although the post office is the cheapest place to get them in most cases.
…edited to add a link to the stamps gallery of Hrvatska pošta: