From Amazon
GP Batteries Primary Batteries GPA76 Alkaline 1.5 V Non-rechargeable Battery – Batteries (Alkaline, 1.5 V, 11.6 MM, 11.6 MM, 5.4 mm, 1.9 g)
Conrad Electronic
GP Batteries Button cell LR44 1.5 V 1 pc(s) 110 mAh Alkali-manganese GP76ASTD967C1
This product is also available at 6 unverified merchants.