Buying keycaps online
Here is a list of shops I looked at when buying keycaps online.
Japan
Yusha Kobo
A well-known keyboard specialty shop in Japan. If you want to buy anything related to custom keyboards, this is a good place to start.
https://yushakobo.jp/product-category/keycaps/
TALP KEYBOARD
They sell individual keycaps in DSA profile and XDA profile.
https://talpkeyboard.stores.jp/?category_id=59be183f428f2d49120007b1
Yukari Keyboard Factory
They mainly carry keycaps from the manufacturer Tai-Hao.
International
AliExpress
A major e-commerce site run by the Alibaba Group. It seemed to have the largest variety of keycaps I found. Searching for something like "keycap dsa" lets you filter by profile, which was convenient. However, OEM profile keycaps also appeared in search results, so check the product description to confirm the profile.
https://ja.aliexpress.com/wholesale?catId=0&SearchText=keycap
Tai-Hao Shop
The official online store for Tai-Hao, a Taiwanese keyboard manufacturer. They have many colorful keycaps, and backlit-compatible keycaps were organized into a separate category, which was helpful.
I didn't look at all their products, but OEM profile seemed common.
https://shop.tai-hao.com/categories/backlit-keycaps
zFrontier
An international shop that offers group buys. Group buys can take several months to ship and may not proceed if they don't meet minimum order requirements, so it's not ideal if you want to buy immediately.
They also have some IN STOCK items with fewer varieties that can be purchased right away.
https://en.zfrontier.com/collections/keysets
DROP
They have a large selection. If you want to filter by profile, searching for something like "cherry keycap" works well.
https://drop.com/mechanical-keyboards/wdrops?subcategory=keycaps