I usually buy my packaging things from stationary stores, but sometimes I reuse cards/repurpose albums and their inclusions to use as postcards! I buy most of my photocards from instagram, tag accounts for your group you want to buy photocards from are usually the best place to start c:
id say look for tag accounts and make mutuals!! To find tag accounts i usually just look up (Group Name) Tags. (Ex. @ateeztgs or @7dreamtag) i mostly find moots and goms from my insta discover page or from story reposts from my moots!! You can also find moots from biasroom a lot of people have their instas linked!!
i like to buy my supplies from Mandu Deco and MeowCafeShop, and ofc re-use stuff i receive. you can also find cute washi-tape from Pipsticks or just on Etsy.