Batteries - some with special max. voltage (LG, Samsung). Use BLF so save 5%.
Samsung ICR18650-32A 3.7V 3200mAh Rechargeable Li-Ion Batteries (2 pieces), Battery Maximum Voltage 4.35 V, unprotected, flattop. Price for a pair $13,97. My link - non - aff-link
Samsung INR18650-29E 3.65V 2850mAh Rechargeable Li-Ion Batteries (2-Pieces), Battery Maximum Voltage 4.2 V --> the charging voltage should be 4,2V +/- 0,05V (from the datasheat of Samsung), unprotected, flattop. Price for a pair $11,85. My link - non - aff-link
LG ICR18650E1 3.7V 3200mAh Rechargeable Li-Ion Batteries (2-Pieces), Battery Maximum Voltage 4.35 V, not real flat top, not button top - I would say a little bit button (not much). Price for a pair $13,49. My link - non - aff-link
32A 29E LG
Sanyo UR18650RX 3.6V 2000mAh Rechargeable Li-Ion Batteries (2-Piece) 22A continuous discharge.Battery Maximum Discharge: 22A, button top, unprotected. Price for a pair $12,23. My link - non - aff-link
Sony US18650VTC4 3.6V 2100mAh Rechargeable Li-Ion Battery (2-Piece).Battery Maximum Discharge: 30A, peak 60A. Max. charging: 4A, flat top, unprotected. Fasttech wrote: Max. battery voltage 4,25V, but no other shop wrote this. All warote 4,2V.Price for a pair $11,87. My link - non - aff-link
ICR26650 KingKong 3.7V 4000mAh Rechargeable Li-Ion Batteries (2-Piece and a box).Battery Maximum Voltage 4.2 V, flat top, unprotected. Price for a pair $18,09. My link - non - aff-link
Wallbuys sells KingKOngs ICR26650 with 4000mAh one piece for $7,26 since a few months. I don't know, if Wallbuys have older stocks (like the LG 18650 D1) of the King Kong and this cell came without a box.