I've dug around for this exact thing more times than I'd like to admit, and I can share a practical path that usually gets me crisp Shinji chair PNGs. First, if you want the clearest frame straight from the source, grab a legally owned digital copy of 'Neon Genesis Evangelion' or 'The End of Evangelion'—Blu-ray rips or official digital purchases give you the highest base resolution. Use a media player like VLC to take a snapshot at the native resolution, then open that image in Photopea, Photoshop, or GIMP to carefully cut out the chair and character. If you prefer not to manually cut, drop the image into remove.bg or a similar background-removal tool, tweak the mask, and export as PNG.
For ready-made transparent PNGs, check fan-art galleries and PNG repositories (DeviantArt, Pixiv, CleanPNG, PNGTree) and smaller image hosts on Twitter or Tumblr; just search for terms like "Shinji chair PNG" or "Shinji Ikari transparent." Reverse-image searches with Google Images or TinEye often find higher-resolution sources of the same edit. If the file you find is low-res, upscale with waifu2x or Topaz Gigapixel AI and then clean edges in Photopea.
Finally, mind permissions: many high-quality PNGs are fan edits so credit the creator and ask before using them commercially. I’ve found this combo of source-capture, smart removal, and gentle upscaling gives the cleanest results, and it’s oddly satisfying to make a perfect transparent Shinji for stickers or headers.