How sure are you, that your default fonts got replaced... *hrhr*
Nowhere in system land as far as I know. But to troubleshoot, use the Type light font editor (Windows) to edit the Metadata of your fonts. Change all "Name fields" (also the ones under advancd) to one and the same simple Name starting with a Letter from A-G - also be sure, that the naming scheme of the four files per font (regular, italic, bold, bolditalic) is exactly as ist should be (search this forum to find the correct naming pattern) - otherwise a custom font simply wont show up using the ;fc-cache method.
It might be as simple as that.
Removing the fonts from the fonts folder in userland and executing ;fc-cache again will "uninstall" custom fonts - so it might very well be that they are just stored in cache - as the ;fc-cache command suggests..