Amazon doesn't sell or support replacement batteries. What makes you think the battery is failing? Unless it's been damaged (e.g. stored empty, left in a hot car), the battery should last around 5 years or so. It's more likely there's something else going on.
Not something to be attempted by anyone not an experienced electronics tech and an experienced electronics tech would know how to find that info. (Or in this case that original parts cannot be had and only third party solutions that may or may not be full of styrofoam packing instead of battery cells are being sold or which have been cannibalized from other dead devices).
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My wife's kindle fire hd8 6th gen is also having issues with the battery. So I disagree with he above saying it should last 5 years.
She treats that thing like it's made of gold ... it's never been over heated or mistreated.
Time is less relevant than the number of full charge cycles. Someone that uses a device with a lithium-ion battery. A typical lithium-ion battery will sustain 300-500 charge cycles. If someone is using it heavily enough they are charging it once per day, a battery lasting less than 3 years would be normal, not unusual. In contrast, for someone that uses it so infrequently they charge it only once per week, the battery should last for years.