Obtain files from other users of FieldWorks, OpenOffice/LibreOffice or other Hunspell-enabled programs, or download them from https://software.sil.org/fieldworks/download/spelling-dictionaries/.
Please help us populate this site with a good supply of quality dictionary files.
Copy the files to make them available to FieldWorks.
These sites have offered them in the past (but may not anymore) - Beware of viruses, adware and spyware when you download from the Internet.
If you have a recent version of LibreOffice 4 installed, and spell checking is working there, you can copy its spelling dictionary files. They may be here:
%appdata%\Roaming\LibreOffice\4\user\extensions\tmp\extensions (Windows®)
/usr/lib/libreoffice/share/wordbook (Linux).
If you download *.oxt files from a LibreOffice you need to change the filename extension to .zip. Then search the .zip file for the desired *.aff and *.dic files.
http://hunspell.sourceforge.net/
https://sourceforge.net/projects/hunspell/files/Hunspell/Documentation/