That's right. No one has EVER created a complete set of orchestral instruments; there’s always a few missing (e.g euphonium) and combining pieces from different authors, it's just awful...

Thanks for the suggestion, Andrei! It's a good one.

In the meantime, I'd recommend checking out the Record Shop in the Retail Stores Kit. There are some musical instruments and related props in there.

Or you can try filtering by Arts/Music under Props in the Object Type filter in Cargo.