You do not need to own the Mausoleum before you can win any key fragments.
With every new unlocked enemy, you have a higher chance for a Fragment. At Old McDonnell's Farm it's chance is similar with a Brain, but from level 26 and above it has the biggest chance from all spoil items.
Text: If you get enough of these, you might just piece something together.
Other notes: Listed as a Boost Item