You can download the Spanish Thesaurus
text file that the Spanish OpenOffice.org Thesaurus is based on from
OpenThesaurus-es. (You'll need
7Zip to unzip the archive.)
Depending on your technical skills, you could either convert the text file to an ebook with
Calibre or first convert the text file to a tab-delimited text file and then convert it to a Kindle dictionary following the instructions on this
web site.