Seeing as people keep asking about where to find cards, I'm going to re-post an edited and updated version my guide from several weeks back.
Be sure to read the entire guide all the way through.
IMPORTANT: This guide is only about the cards and how to access them. Cards will only work in a .png file format and in the card format (you should be able to discern the difference between a card and not after looking at them for awhile).
This is the path for where to put your cards. Female go in the female subfolder, males go in the male subfolder. Fairly certain traps go in the male subfolder as well(?)
If you create a subfolder within either the male or female folders, the game will not be able to read it for use in the main game or for free-h mode. However, they can be read in the character creator which makes it easier to edit cards en mass until you're ready to actually use them.
Option 1: From the Official Server
In earlier builds, it was possible to access the official illusion server in game. However, around January or February, Illusion made some changes so the server that changes how it can be accessed. Based on the testing I've done using the repacks, it would appear the official server is only accessible internationally if you use a VPN to change your location to Japan. Unfortunately, even this solution is partially effective. Most likely, there's only a specific window of time it can be accessed. So, what else can we do?
Option 2: Official Server Cards Re-uploaded
Since we've run into the issue of not being able to acquire cards through the official server, we are stuck looking elsewhere. Thankfully, hongfire user screwthisnoise (the same poster of the R# repacks) has shared a mega folder containing all of the cards from the official server (and from other illusion games).
You must be registered to see the links
Now, this folder is updated periodically. But that isn't really a problem considering there's over 60000 cards for females alone. This way, you can access most if not all of the cards off the official server.
Note: Most, if not all of the cards on there should work with the R6 repack or whatever is the current version.
Option 3: Sites for content creators
Some creators refrain from uploading their creations to the official server. This can be attributed to either using many independent mods or the limitations stated under option 1. There are two popular sites for content creators that are easily accessed.
The first is booru:
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Featuring the same layout as many of the anime/hentai sharing image boards, booru has a simple layout which makes it easy to look at specific creators, character features/traits, and series tags.
The second is kenzato.uk/booru:
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Considerably different from the booru set up, kenzato allows you to browse by popularity, likes, chronological, and by user. Unfortunately there are no tags, but most of the regular users/creators are smart enough to create folders for specific themes or series.
IMPORTANT NOTE: On both of these sites, the creators may be using mods not featured in the repacks. As such, be sure to check the descriptions to see if the author list what mods and where to find them.
Option 4: Other creations (For advanced users)
For those of you who have exhausted the aforementioned options, or you are looking for a specific creation, there is the option of using pixiv:
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You will need an account for this site to access R18 content. This site admittedly has a bit of a learning curve if you don't speak Japanese. First and foremost, you'll need to understand that not all posts/creators for koikatsu will feature downloadable cards. Some creators are rather possessive about their creations: many will provide links to download, while others will hiss at you like a racoon protecting some scraps of food. Tread carefully.
The best guarantee to find cards to download is to search the tag
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which translates to "character release (Koikatsu)". Because of the way pixiv recognizes tags, typing in the english version of the tag will result in it failing to process.
Once you find a post you like, the author may have the card available in post as a .png which can be right-clicked and downloaded. Many authors use either googledrive, twitter, or uploader.jp to host their cards externally. Carefully read each post and each author page to figure out their preferences.
(NEW as of Oct. 5th, 2019) Option 5: Discord
After rereading this post, I forgot to add the discord link for the koikatsu group:
You must be registered to see the links
For those who do not know what discord is, essentially it's a combination between a chatroom and forum, including modernized elements like VOIP. Primarily used for gaming groups, it has grown to support many communities and is used as an alternate to apps like teamspeak.
This discord server has many users active, including those from the aforementioned options, which allows you to potentially find cards you missed or even some unique creations not posted elsewhere.
Note: The server has recently cracked down on certain "extreme fetish" material, so read the rules before posting.
Once you have entered the group (or room), select #card-sharing from the side panel titled "Sharing Site". Then simply scroll through and download the cards you like.
ENDING Bonus: Many authors create their cards to be used just within the character studio, so their sexual traits are all set to "no". Any cards can be loaded within the character creator and re-saved with the changes made. Additional changes may need to be made to certain accessories as they may be set to "main" instead of "sub". Ideally, any large accessories need to be changed to "sub" so they do not show up in H-scenes.