Are they paper publications? Like those doujin marts?
Some are, some are not.
Most of this kind doujin can either be found at events like GLFes (which is in next month I think?) and/or comic shops of different kinds. I haven't gone to events much so I buy mine mostly through Comic Zin or Toranoana, which are both physical retail stores with online shops (Not sure if they ship internationally though).
I was wondering maybe the reason why Aki is dreaming about those things is because Saki has a strong desire to fuck her friend? because something like this happened to me. I have a co worker and we are like best of friends and she claims that we are soulmates then one day she got promoted in our work and I was desiring to buy her a necklace to congratulate her but I couldn't find a perfect necklace for her, then few days after (me still looking for a perfect necklace for her) she enthusiastically told me that she had a dream that someone put a heart shape necklace on her and that's when I got the idea that I should get her a heart shape necklace.
and tbh this is not the first time that this happened, well, we often talk about dreams and such and she always dream about the things that I desire or predicts the things that happened to me. it feels like she has a sixth sense when it comes to me. :/