I wondered what the various collections of easy trombone music are like; things like "Time pieces for Trombone" or "First book of Trombone solos" and whether teachers use them with their pupils.
My daughter started the trombone with a de Haske tutor book (I think in English it's called "Read, Listen and Play") which she used for about a year and a half. She has now progressed onto "real" music and I have to buy each piece individually. They are all pieces by contemporary French composers which with postage (I have failed to find an online site which sells them postage free) work out at between 15 and 20 euros each. Not only am I starting to find it rather expensive but they are all very similar (despite being by different composers). They are all in three parts with a slower middle section (which is often a waltz and often involves a key change) and all the piano parts rely heavily on octaves in the left-hand for their harmonisation.