Even if it's a grindy way, it would be nicer to have indeed another way to get the notes... Being stuck with still 1 note left and unable to get it after tens of hours is frustating...
I'd love some king of "field note fragments" kind of way :
On each Skirmishe/Duty where you can get a specific note, you have a chance (higher than the full note) to get a note fragment, and you need <x> fragments of said note to complete it.
Give it 5, 10, 20 fragments required, with a decent drop rate (20-30%) so this is giving those who are spending(wasting sometimes) their time to get that mount a B-route to look into if A-route (full note) just doesn't do the trick

Yes, you have the Duel to get some of them 100%, but come on, not everyone is able to clear it (and being able to try them is time consuming), not to mention the required preparation...

I suggest the fragments thing cause using Coins could be a bit "weird" to trade from the basecamp (like, the guy is selling notes?! Where did you get them !). I know logic has its limit in the game of course, but let's try not to add more "weird logic" than there are atm.