Ask Slashdot: Where Can I Find Good Replacement Batteries?

An anonymous reader writes: I have an old phone with a battery that barely works anymore. My current phone's battery is mediocre — I can put up with it, but I've been thinking about getting a new one. My four-year-old ThinkPad holds less of a charge than I'd like, and less than it did when I bought it. In all these cases, the only thing holding me back from buying a new battery is that I'm not sure where to find a good one. Searching for my phone's battery on Amazon (or any major online retailer) yields a dozen results, all fairly cheap. But which are reliable? They all seem to have varying reviews, ranging from "Perfect official factory replacement!" to "Garbage knock-off, worse than the battery I replaced." Part numbers don't seem to help, as the knock-offs replicate those pretty well. I ask you, Slashdot: where can I find a good replacement battery?