Thanks a lot for the tip, i'll tell him tomorrow
Effectively, as JohnL says, we tried a 0.487 bore inner from another trombone and it didn't even fit in the outer slide... Now we try to get something from Kühln & Hoyer, as we are in Germany. Otherwise, he knows a company than can pull inners of every custom size, but they cost 300€ for both... For a 200€ trombone it's a bit much but well, it's my main instrument now.