@FloraMae :-
Where missing dependencies can't be found in the distro repositories, THEN it's time to go "lib-hunting". You shouldn't just give up. I know that source code for dependencies is usually compiled against the specific 'tool-chain' FOR that distro, but it IS entirely possible to "borrow" the same dependency from other, unrelated locations. Sure, it won't be an 'exact' match.......but 9 times out of 10, it will satisfy the required functionality of the application in question.
As for .debs, .rpms, .txz, etc.......these are all packaging conventions, assembled in specific ways to work with particular package management systems. At heart, different packages of a particular dependency will have all been built against the same source code.....just packed in a slightly different way as the final step.
This is a good place to go "hunting":-
https://pkgs.org/
Or, you could try here:-
https://old-releases.ubuntu.com/ubuntu/pool/
.....although it can be a bit of a PITA to navigate around. This is the 'master' repo; here, Canonical keep every package - application, dependency, framework, whatever - that has ever been built for Ubuntu since the first release back in 2004. They haven't thrown anything away.
Mike. 