Hi. Air battery costs around 65-75 € and you can find them in Amazon, in eBay or in your local RC hobby store. In case you'll be interested rebuilding one you should find 3 LIFE battery elements 3.7v each.
Instead you can open carefully the dead battery, remove the dead elements and keep the charging circuit. On this circuit you can see the points that the two terminals are connected (red and black thick wires). Removing elements means you are going to come off (unsold) these wires. In their position you must get soldering another red and black wire fixed in an XT60 or Tplug connector (depends on which of them is used in a brand new lipo battery you are gonna buy). This battery should be at least 11.1v 2800mAh 30C. And naturally you will attach it externally using a strap or something like this because their size is longer than the original Dji batteries. The result would be at least the same as with the original bats or even better in flight time if you use bigger battery in mAh capacity (3300mAh).
Keep in mind the actual size of the new battery not blocking the downward sensors.
Hope this helps.