I was doing a project on Binary stars. I wanted data of masses, eccentricity, semi-major axis, and radial velocity. I tried many sites but I couldn't find them. Where can I download such data from?
If you are looking for professional data you can find them on VizieR. If you search for double star you will find 103 catalogues on double stars (some containing data only for a specific area in the sky). I haven't looked at all 103 catalogues, but the Orbits of 451 wide visual double stars : J/PAZh/45/35 looks promising. (If you want to download it, go to the FTP tab.)