YOU may have luck finding a disk-loadable version of Star Raiders. I understand how hard it is to get ahold of Atari 8-bit software these days.
Before you try to find one online, I suggest you first trying to find an actual cartridge copy of the game. Visit your favorite Atari 8-bit vendor.
If you're really interested in downloading a copy, you've been warned. Try doing a search on Atari 8-bit sites using XL Search, the Atari 8-bit FTP search engine. Search for "starraid"