SARK-110 original battery model is PR-384160N; see datasheet at the end of this page. The basic specifications are the following:There are some vendors of Li-Ion batteries offering similar batteries. These batteries are normally supplied without a connector, so it will require the original connector being cut off and rejoined to the new battery.
This higher capacity battery is slightly thicker than the original one, but it fits inside the SARK-110 housing.
This higher capacity battery offered by Adafruit is slightly thicker than the original one, but it fits inside the SARK-110 housing.
Note that the provided connector is not compatible; also check shipping restrictions