I'm watching GetBackers and after the fight against Makubex in Infinity Fortress i only find fillers and story with no plot-related history. Which ones are the fillers and which correspond to the manga?
After the point you mentioned the content is basically entirely filler starting with episode 26 (the beginning of season 2 of the anime). Other than the final arc (episodes 43-49) which provides some conclusion to the anime, it is episodic content. The final arc is anime-original and quite a bit different from the manga.
A few of the episodes of the anime are based on interlude manga chapters, but they're still episodic and don't forward the plot. These episodes were expanded quite a bit from the manga chapters which were comedic interludes that did not forward the story significantly. Episodes 26, 38, 39, and 40 are the only examples of this I know of, though it's been a while since I've read the manga or watched the anime so I might have missed one. These episodes still have a lot of anime-original content, so if you don't like the filler at all you probably won't enjoy these regardless of whether they're based on the manga.
So, in short, if you want the continuation of the GetBackers story, the only real option is to read the manga. The anime episode 25 corresponds roughly to chapter 83, the end of the "Return to Infinity Fortress" arc. The anime does have a bit of manga content after that point, but I doubt it would be worth watching if all you want is the story and you don't enjoy the filler episodes at all.