If you tell me (e.g., a specific forum, file-hosting site, or torrent), I can try to help you find the correct search query to locate it.
If this is a large download, it is usually found on file-sharing sites, torrent sites, or specific forums where the original user posted the links.
Since I cannot directly browse or download file-sharing sites to find specific file parts, you will need to check the original source where you downloaded the other parts.
(like .part1, .part2, etc.) must all be in the same folder to be extracted properly.