Well, that kind of depends of your taste in music, but you'd have to buy them or go the illegal route. The latter would mean you would often receive poorly ripped tracks, tracks which have been transcoded, tracks which have been ripped without any sort of control (resulting in beeps, ticks, and whatnot).
The former would most likely make it just as expensive as buy new ones on CDs, but resulting in the same ultimately non-future proof MP3 or MP4 quality, not to add that most of the MP3/Mp4/WMA music you can buy is DRM'ed (which you don't want it to be if you think about it).
If I where in your situation I would keep the lot "as is", and then rebuy some of the CD's (perhaps second hand?). I would propably not rebuy albums with things like Dead Kennedys, but Primus and my operas and jazz albums would certainly have to be rebought.
In other words, what you did back then when you threw out your albums have now caught up with you (dum dum dumdum ("jaws" theme, lol)). It always pays to think about what will happen in the future.
If it helps, all the people who have bought into the whole iTunes-deal with DRM will be where you are in the future. But unlike you, they won't be able to continue their use of the music if they want to use a better (or just another) player without having to transcode resulting in even worse quality.
Edit: Whoah! That was a bit longer than intended