From what I can find online, open/libreoffice uses Hunspell which is compatible with (derived from) myspell. You can find system-level installed dictionaries in
/usr/share/hunspell
/usr/share/myspell
/usr/lib/libreoffice/share/wordbook
(I would start here)
Here's a related post from askUbuntu on how to install additional dictionaries, providing some additional insight on the subject.
You can also get the source files here. Look under libreoffice/dictionaries
.