There are two free de-ess plugins I know of which work in Audacity …
Spitfish … http://www.digitalfishphones.com/main.php?item=2&subItem=5
and Gmulti (multiband compressor) … GVST - GMulti [just use the high freq “band3” to de-ess]
Spitfish is available for Windows and Mac Operating Systems, GMulti is Windows only.
[ I couldn’t get the free “mda” De-ess.dll to work effectively in Audacity … Missing features - Audacity Support , however most of the “mda” VST pack do work for me ]