If you have never been in Prague, you will find that all old churches are amazing organs. So you just have to walk in and check it out. It is usually on upper floor, visible from main area, but not always accessible. Few to mention: Sv. Mikulas (Old Town Square), they have concerts there for tourists every day, but not always with organ. Sv, Vitus Cathedral (Prague Castle (free entrance for main area downstair, fee for further), Another one, which was always very popular with locals is short walk from Old Town square and even under communists there were organ concerts of very high quality - Kostel U sv. Jakuba is open and you can come in, listing of concerts is on the board outside. For more see Prazske Jaro http://www.pis.cz/en