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Do not get confused by the name, Student Agency's bus lines and other services are available for all customers, not only for students or youth. (In some cases, there are discounts for ISIC holders or young people of age <= 26, though.)
Student Agency operates easily recognizable yellow buses, see [picture 1] and [picture 2].
They offer a possibility to buy tickets for the bus lines they operate online; unfortunately, this is available in Czech language only. But adventurous people can follow this manual. :-)