We bought them frozen from The Fresh Fish Place in Port Lincoln.
I Googled to find a few recipes ...
http://www.aww.com.au/food/recipes/2008/2/tempura-soft-shell-crabs-with-aioli/
http://www.sbs.com.au/food/recipes/tempura-soft-shell-crab-fried-chillies
to get some ideas.
If you need to clean the crabs here was a short You Tube clip ... I discovered that we needed to clean ours :-)
https://www.youtube.com/watch?v=Ytfb5yal5pM