Grade 5 ceramic balls. Sold by the unit. Meaning, you need one of these for each side of your hub.

Measurements:
• #6902: Internal Ø 15mm; External Ø: 28mm; Thickness: 7mm.
• #6903: Internal Ø 17mm; External Ø: 30mm; Thickness: 7mm.
• #6802: Internal Ø 15mm; External Ø: 24mm; Thickness: 5mm.
• #6805: Internal Ø 25mm; External Ø: 35mm; Thickness: 7mm.