If you need stamps, the easiest option is to buy them at the post office (pošta). It is easily identified by the yellow and blue logo (yellow postman’s trumpet against a white background).
Post office in Prague city centre
Jindřišská 14, Prague 1 – New Town (Nové Město)
Opening hours: Monday to Friday from 6 a.m. to 10 p.m. and Saturday from 8 a.m. to 6 p.m.
Usually you can buy stamps at a newsagent’s (trafika), a shop or often a kiosk. Bear in mind, however, that you won’t find them at a trafika in a city centre.
If you want to send a postcard or letter (weighing up to 50 g):
Post boxes are painted a vivid orange, making them easy to spot. You’ll find them at various locations in Prague, especially by the entrances to post offices. There are also post boxes at Kaprova street 12, just a few metres away from the Old Town Square, and opposite the main entrance to the Municipal House, Republic square 103 (Náměstí Republiky).