You will probably find it on ModBD, though be warned, current version of Discovery will automatically delete your save files for compatibility. You will have to back up your saves every time you exit the game to avoid losing them, even if they're in separate install locations. iirc it does this every time you open the launcher
Though I can't remember what version of Discovery lets you play through the campaign. I know 4.85 doesn't, so you should check 83 or 84
Well after a dozen try with different versions, it doesn't even load properly and when I exit the game FMM says it can't access freelancer.ini (and can't even disactivate/delete the mod after that)