The root monsters should respawn in the same swamp after 'a few days'; you'll
eventually get all you need to fully upgrade a root set by visiting a few swamps
in rotation; you're going to want to farm leeches a bit anyway.

The Leviathans can't really be farmed. You just have to find them. In the ocean
somewhere or close to shore... either way. They don't appear to respawn
though someone claimed a long time ago that one did respawn in the
same spot but it took a looong time to happen.
You don't really need that much chitin to make and upgrade everything;
I think 3 Leviathans is enough.