There are 900 total notes in Showdown though they are valued at 1 (bronze), 5 (silver) and 10 (gold) so you don't have to actually collect 900 total notes. Check your progress through the Statistics tab in the save menu. Some hard to find notes:
- Underwater beneheath the pier where doors 3 and 4 of Banjoland are.
- Underwater beneath Klungo's Arcade.
- On the island behind the Orange Jinjo house.
- Under the bridge leading to the Orange Jinjo house.
- Inside the three shacks on the beach near Klung's Arcade.
- In the alley between the Jiggoseum doors.
- On a small walkway above Humba's shop.
If you're stuck at something like 899/900 or 890/900 you most likely missed one note from a larger group of notes. Most people tend to miss a note underwater since they are tough to see. Check those areas over again first.