You can also go to your friendly HotSpring Portable Spa dealer and purchase one of three things:
Vanishing Act regular and eXtra Large (a water softening pillow which you set over your water return fitting and allow to stay there for 24 hours. Depending upon your spa and it's circulatory system, it may take longer - the HotSpring tubs have constant circulation and it works in one day, but if your spa starts and stops the circulation it may take longer.
Also, Watkins has announced the arrival of a portable softener system for HotSpring dealers. We can loan you this small softener, and you use it to fill your tub. Not free, but easy.
There are also systems available from West Marine and other places which come with easy garden hose connectors -
do a Google search for 'Portable Water Softener' and you will see lots to choose from
HTH