Lithium is the most common type of rechargable battery used in consumer electronics, there are two types of lithium batteries that are used, these are lithium ion and lithium polymer, don't get these confused with no-rechargeable batteries like 9 volt, AA, AAA and some coin cells which are all made of alkaline if you attempted to charge these you'd not have a fun time. Tones of electronic devices use these batteries but the cheapest places to find them is in old laptops. If you go to ewaist facility and ask them for laptop batteries you can get them for reeeeeally cheap while you're there you might as well keep and eye out for anyother electronics that look like they might have a battery inside such as
Cars ( please dont take apart a tesla :( they are too pretty)
While finding these other battery sources is easy i would defiantly recommend sticking to only looking for laptop batteries and they are the cheapest, have the highest quality brands, and the most cells within the battery.
laptops typically use 12 volts as an operating voltage but a lithium cell can only provide and average of 3.7 volts meaning that most laptop manufactures arrange 3 batteries in series which creates one battery with a total voltage of 11.1 volts. But most laptops have 6 cells (for a longer run time) and still have a voltage of 11.1 volts. This is because the cells are divided into couples of 3, these couples are connected in parallel which keeps the voltages at 3.7 volts then the 3 couples are connected together in series to get a voltage of 11.1 volts. Anyway now that we know how they works lets get them out, most older laptops use a lithium ion cell called 18650 which is a very common battery type, some of the Tesla cars actually run on these batteries! The 18650 cells have a metal cylinder shape, they are pretty easy to get out of the battery case all you have to do is pull the casing apart and cut all the cables you can see, you'll see that all the cell are connected together with a thick flat metal connector, we need to slowly and carefully pull this off the battery taking care not to let any pieces of metal touch, if the metal refuses to peel off stop pulling and just cut the metal once this is done you should have a nice collection of batteries, just a heads up most newer and thinner laptops doesn't use 18650 cells because they are too thick so they use a lithium polymer cell if you find these you can use them in the same way as you would use the 18650 just be a bit more careful with them as they don't have any protection around them so its pretty easy to puncture them and cause an internal short circuit.
So now we've got the cells out of their casing and seperated them, this is where we are going to need a multimeter, over time some cells degrade, leak or rustwhich will result in a dead cell. Finding cells that have leaked or rusted are pretty easy just keep an eye out for discussing oozing batteries or rust however finding degraded cells isnt that easy, a cell may look perfectly healthy but is just completely dead, these are the degraded cells we find them by hooking them up to a multimeter and then put it on voltage mode, the positive probe goes to the postive terminal of the cell and the negative probe goes to the negative terminal. If the voltage is bellow 2.5 volts it means its kind of degraded but can still hold a charge, we are going to want to avoid them if possible however anything bellow 1 volt is completely dead and should be deposed of at a battery recycling facility. The ideal voltage you are looking for is between 3 and 4.2 Volts