I tend to use bepis to check for most of the cards i have but also you can browse on pixiv using the specific tag (name of the character/franchise you want to search and the koikatsu tag together) for it and you can find lots of stuff if there are creators that share those cards
Koikatsu Cards: The Best Kept Secret In The Industry.
I tend to use bepis to check for most of the cards i have but also you can browse on pixiv using the specific tag...