According to Mark's answer to this Drones and Model Aircraft SE question, I likely need to buy quite a few replacement LiPo batteries.
Finding large LiPo batteries for expensive aircraft is usually rather simple. There are scores of websites that sell them, as well as many brick-and-mortar RC/hobby stores. Standards exist to help guide one's purchases.
But for the small "non-replaceable" batteries that are included in inexpensive RC aircraft, what are some good sources? By "non-replaceable", I mean that the batteries are not designed to be replaced, but, of course, can be replaced by someone experienced with electronics and aware of the required safety protocols.
What methodology is appropriate for getting the right battery for each aircraft?