Please

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“𝒯𝒽𝑒 𝓇𝑜𝓈𝑒'𝓈 𝓇𝒶𝓇𝑒𝓈𝓉 𝑒𝓈𝓈𝑒𝓃𝒸𝑒 𝓁𝒾𝓋𝑒𝓈 𝒾𝓃 𝓉𝒽𝑒 𝓉𝒽𝑜𝓇𝓃𝓈.” ― 𝑀𝒶𝓌𝓁𝒶𝓃𝒶 𝒥𝒶𝓁𝒶𝓁-𝒶𝓁-𝒟𝒾𝓃 𝑅𝓊𝓂𝒾.
あたしは腐女子です。少女漫画、たくさん読んでください。

The best way I have found to search for specific doujinshi pairings, is to go to "Genres" search, select "Doujinshi", then when you get the results, go to the bottom of the page where it says "Title", click on the drown-down button and switch to "Description", then in the box type the pairing you want - "germany italy"

It sounds complicated but it really isn't. I don't know if there is a better way.

You could find all Hetalia doujinshi by doing a search for "Hetalia dj", but you won't be able to screen by pairing that way.

I get all of my doujinshi needs from tumblr. You can read GerIta doujinshi here, here, here, and here.