Where do you look for Library Upgrades?

jjohn asks: "It seems simple enough. You want a new linux toy. You find the source and download it. You do a 'configure', you type 'make' and then ... poof. You're missing a library file. Is there a good text on upgraded system libraries? Is there one or two places, besides old sunsite.unc.edu/pub/Linux, that are "main" repositories for Linux libs? I prefere to compile source rather than get binaries. For the record, I'm looking to upgrade the libs on my old Slack 3.0 box...Thanks"