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 below, or [picture 1], or [picture 2].
There is a possibility to buy bus tickets online; unfortunately, their e-shop is available in Czech language only. But the adventurous can follow this manual. :-)