The capacity of general 26650 batteries is between 4000-6000 mAh, but not all batteries can do this, because if the quality of the battery is good, it must not only have high capacity, but also be well made. Additionally, choose Tanke. Such branded products are also indispensable. It is relatively better to buy a better brand.
There are not many manufacturers producing 26650 batteries, and there are even fewer major brands. It is not like 18650, which is used as a large-scale lithium battery for electronic products. Sanyo, Panasonic, Sony and Samsung do not produce it, others. It's not considered a brand, is it?
Actually, you can use 18650 and add an adapter ring to replace the 26650, but the discharge current should not be not too big