Physical doujin in english is usually bootleg/illegal distributed japanese doujins by people who take the scans online and translate them and then print them. Altought I know they exist, I do not not know where to get them, in some conventions they may do pop up, but I dont recommend and most of them have low quality due to change of sizes of the original works, as well as bad paper and so. You may better save your fav doujins from an online web and go print a copy for yourself instead, rather than buy bootleg books, but I wont judge if you buy bootleg doujins either. I always buy them in japanese from japanese stores to support the artists, even importing from Japan is cheap now and it really worths the money. Even for small quantities of copies, japanese artists do spend in making their books, and I have some with more than 20 years that are radioactive white because seven in second hand, japanese collectioners do treat their stuff very well. You may get a haul in a lot of auctions and shops, back in last month Surugaya had 100 H doujins at 20$ (blind lucy box) for a limited time, for example. If you don't mind second hand stuff, I really recommend it search around.

Have you tried Fakku.com they sell physical copies of English tranlated doujins.

By the way the books are not bootleg or illegal. They publish the doujins officially. Here is a link External

I said usually, not always, but only a little amount of them are aproved by the author for releasing outside Japan translated. The most stuff that comes out, original may be licensed, but parody books rarely make a legal translation since in Japan they preffer to keep fan activities a bit more closed to avoid any bad looks on their culture or the original series damaged. There are still parody doujins which make out of the country, but its a very little amount, and the most of them have been without permission. Some cases are conventions books, but for example, some south America's countries had Dragon Balls doujins published as a "serial" for adults, and here in my country there was several hentai magazines wich had full doujins with no permission at all. I have known that at least Naitou2 has released its book with Fakku legally, but Im browsing and most of doujins are only digital, the original mangas does include paperbacks. in most of them. So for the author of the post, it's not much to choose from. A good way to arrange this could be sites like Fakku or DLSite adding a paperback system, but most japanese authors prefferto mantain their works closed to the outside, so it would be hard to make a service like that. Many illegal or lack of respect activities towards doujinshi community from the west has made this a bit difficult, so at least in terms of variety, there's not much to do that copy for yourself or keep with the original in japanese.