I haven’t settled on a specific place yet, as I’m only a new buyer, and haven’t figured out which is best (cheapest!) yet lol From what I’ve found, they’re all from Japan; but there are both Japanese and English websites to shop from (however, all doujinshi is in Japanese).
All the Japanese creators I’ve found on Twitter sell their stuff here: https://www.melonbooks.co.jp/fromagee/ It is 100% Japanese, and you have to use Japanese on it; but google translate (which automatically comes up for me to translate the page) makes it readable enough to navigate & find out what the stories are about. Sadly, the company will only send stock to Japanese addresses, but there is a way around that by using a third party service.
The first time I bought doujinshi, I used a warehouse service where you tell them what you want, and they buy it for you. It then gets shipped to their warehouse in Japan, and from there, they post it to you themselves. The cost isn’t too good, though, bc you have to pay for the doujinshi + shipping + fees for using the warehouse (some of which I wasn’t aware about until I’d already paid for the doujinshi). In total, for 2 doujinshi, I spent about $60.
My latest order (which just arrived yesterday! Yay!) on a different website (https://buydoujinshi.com/) was even more expensive because there was only the super expensive shipping available due to covid; but they did offer to keep my stuff in their warehouse until a cheaper option became available. My shipping was cheaper with the warehouse (also during covid) because it used the really slow shipping that takes like 3 months to get here as an option; but I’m guessing this company doesn’t use it.
buydoujinshi is in English, but if you want to find exactly what you’re looking for, whether it be a certain ship or doujinshi, you’ll have better luck using Japanese, so it’s best to either copy & paste what you want from link 1 into link 2’s search bar, or the ship/ names in the search bar (I’ll write them in Japanese below so you don’t have to deal with google translate!), and you’ll get plenty of results!
kakairu: カカイル
Hatake Kakashi: はたけ カカシ
Umino Iruka: うみの イルカ
Normally, there’s no space between the first & surnames, but I’ve done that in case you want to search a name separately
I’m yet to try otakurepublic (link takes you directly to doujinshi with the kakairu tag), but it is completely in English, so you won’t have to worry about copying any Japanese. They do new & used stuff; and although the prices seem steep compared to the other places (I also have a sneaking suspicion it’s in USD, so keep that in mind), shipping is free. Only thing is, I can’t find a description for anything, so that is something you’d probably have to look up elsewhere. And I would definitely recommend doing that, bc some things tagged as doujinshi is just writing; and if you can’t read Japanese, you’re gonna be screwed XD
That’s my experience so far in buying doujinshi. I hope the info helps!