Hi,
Here is a link that shows how to replace the battery in your tablet.
https://www.youtube.com/watch?v=N6AMRy4di0s
They also advertise that they sell batteries for the tablet. You may wish to 'Google' search for'' Dell Venue 8 Pro battery'' to see if there are other suppliers and if the price being asked for the battery by the people in the video is competitive.
https://www.parts-people.com/index.php?action=category&id=154&subid=449&refine=battery