It's been a long time since I've been in Prague, but don't the vendors in Wenceslas Square sell normal hot-dog type buns?

http://czechoutchannel.blogspot.com/2007/05/wenceslas-square...

I remember getting American-style buns and the rolls that you're referring to. I prefer the American-style buns because when you take a bite from the roll version, the excess sauce drips out the other side, whereas you can maneuver an American-style bun such that it doesn't drip.