Excellent question, @veemonup, and the answer is, simply, I have a lot of sources. In some cases, such as with Groudon and Hoennian legendaries, I may actually visit sites related to them, accompanied by experts, in order to gather information for myself. Sometimes, it’s better to see the sites in order to understand a legendary’s power, its element, or the vaguest notion of how it lived. Other times, it’s simply better to visit such sites in order to take in local culture and gain a better understanding first-hand of the lore surrounding them.

In other cases, I may travel to regions in order to locate texts related to legendaries (as with the Sinnohan legendaries, whose sacred texts have been collected and stored in the highly venerable Canalave Library), or I may simply dig for scans or transcribed texts online.

Finally, the other option is simply consulting a network of experts, such as Cynthia, Steven, Professor Sycamore, and so forth. Oftentimes, their research is just as comprehensive and invaluable as anything I can find on my own. Perhaps even more so, as they’ve got more years of experience and exposure to those legends than I do, and thus, they’re more capable of interpreting texts.

(Alternatively, in some cases, it’s better to consult the experts than anything else. Take Johtonian legends, for example. While I recommend visiting our shrines and while our texts are highly enlightening if you can read Old Johtonian, it’s just an all-around better experience if you ask a kimono girl.)

Of course, I’m aware that these methods aren’t exactly feasible for anyone with limited time, money, or mobility, and for that, I apologize. I can recommend the second, though—particularly the part concerning finding scans or transcribed texts online. Many libraries in which the old or sacred texts are housed have actually carefully scanned them in order to make them available online (usually because the texts themselves are culturally important but too fragile to display or share with the public). It would be worth it to start there, then use search engines to find more. For example, try visiting canalavelibrary.org if you’re interested in Palkia, then try searching for professors’ websites or other academically recommended text depositories.

Good luck!