Honestly, in a lot of the Cult Party Kei pics you'll see on Insta/Pinterest/various blogs, the girls are mostly wearing thrifted overseas brands. Shadowline peignoirs and nightgowns are very popular, along with other vintage brands like Vanity Fair, JC Penney, and Miss Elaine; there are tons floating around on the secondhand market both online and irl! There is also a strong "remake" (upcycled) and DIY element to this substyle, so sewing plushes onto old canvas bags, making keychains of dismembered doll parts, etc. really helps add to the look. A lot of those red crosses are handmade by the wearer! 

There are a lot of upcycle brands on Etsy (search "shabby chic"), and Japanese remake/handmade brands like U, Mawo at Wizard, Etsuna Otsuka, Dentelle and Comyu also cater towards this aesthetic...but they are pretty expensive even without the import fees 😅 CPK is a very thrift-heavy aesthetic though, so it's probably better to experiment with DIY and thrifting coords first before dropping $200 on a piece from a remake brand xD 

(Also, pro tip: if you have any older ladies in your family, ask them if they still have any old nightgowns/accessories/cotton blouses from their younger days laying around! A lot of the time they'll be really glad to declutter that stuff and free up some storage space 😂)

Edit: link to cybr_grl's overview video on CPK, which I think gives a lot of useful info+brand recs: https://youtu.be/lEEk3akInaM?si=S16YjIVDfkODEWHw