Note that this almost surely needs to be a diametrically-magnetized magnet, not a standard axially-magnetized magnet! I can’t find in the datasheet anywhere that calls out using a diametrically-magnetized magnet, but it does state that “The AS5047P uses 4 integrated Hall sensors which produce a voltage proportional to the orthogonal component of the magnetic field to the die”. If a magnet is axially magnetized, the orthogonal magnetic field component will be near constant, and the sensor wouldn’t work well.
kjmagnetics is where I buy most of my magnets, and has a wide variety of diametrically magnetized magnets, including ones similar to the 8mm⌀ and 3mm height N35H magnets used to characterize the sensor in the datasheet. Since they sell in low quantity, their prices are very reasonable, and I’ve never had an issue with them or their magnets, I’d recommend using them.
Edit: the devboard does include this diametrically-magnetized magnet, which confirms my suspicions.