I found a mirror on the internet archive. Based on this post, it seems official. The archive has both a torrent and a direct download link, so choose whichever works better for you.
Internet Archive link: bazzite-ally : Free Download, Borrow, and Streaming : Internet Archive
Note: The internet archive link seems to be of the previous release of the iso, not the current iso. I think it probably should be fine to try and use this iso and then update, but I cannot be sure.
Alternatively, perhaps try downloading the file with a cli downloader like curl, aria2c, or wget. That or maybe try some sort of vpn?