I’ve seen people buy off mericai (or something like that idk how to spell it) and have had good reviews of it
yeahhh mercari is decent. I just got a Huening Kai one for like $6.
There is also the Japanese Mercari website which can have a lot of things you wouldn't normally find on U.S websites, but you will need to use a proxy shipping service for it.
Ebay can be somewhat decent, but a bit on the pricey end. You can also find people selling their cards on social media such as twitter or Instagram. That can be good for finding harder to get photocards. Like, d-crunch is not a super popular group so finding cards from them is kind of hard to begin with, but I found a lot of d-crunch cards on Twitter and Instagram.