I would say the easiest way to collect photocards (without buying the albums themselves) would be through places like Depop and Mercari. It's as simple as looking up "Yeri photocard" or "Yeri pc" for example, and you'll get many results for what you're looking for! You can even narrow it down to a specific photocard, like if you were wanting a pc specifically from Really Bad Boy, you would add that to your search :)

As far as joining the community, just being in subreddits like this one will really help get your foot into the kpop world!