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).
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 12 Kaprova Street, just a few metres away from the Old Town Square, and opposite the main entrance to the Municipal House, 103 Republic square (Náměstí Republiky).