Repositories like the Offline Eaglercraft Collection on GitHub provide versions that don't even need an active internet connection once the file is loaded.
If you're looking for a place to play, community hubs like the Eaglercraft Subreddit often have updated links and server lists. Just be careful when downloading files from unknown sources, as "re-uploads" can sometimes contain malicious scripts. Eaglercraft -file-
Many school and office networks block gaming URLs. Since you are running a local file from your hard drive, there is no "website" for the filter to block. Eaglercraft -file-