A quick heads up... on my travels around Aliexpress I came across sellers like this:
One interesting thing they sell is kits for building your own power tool batteries. For $15-20 they provide the shell to fit your Makita/Dewalt/Bosch/etc tool, the battery management circuit board inside the battery, and all the parts to put it together. You provide some lithium ion cells (usually 18650 or 21700 size) - either new or harvested from old batteries.
The advantage of this approach over buying aftermarket batteries is that if you buy the cells from a reputable source you know they are good quality, rather than whatever race-to-the-bottom cells the knockoff battery manufacturer chose to fit. You have to work a little harder to fit them though (it helps to buy cells with pre-welded tabs to make the job easier).
I have no experience with this seller or their products, but thought it was worth mentioning. Other sellers are available...
Theo