Most of the larger Bunning's Warehouse stores have compression springs of various sizes. I did a search and found one Aus guy that is using spring C-822 from Bunnings. Search on the Bunnings website and you can find the springs. That C-822 seems a little long to me (at 4" extended) but I guess when compressed it seemed to work for the guy I found in the search. Here is the link, page down to his entry (name is Duffy):
http://www.myastrospace.com/forums/f...upgrade-advise
The Bunnings info:
http://www.bunnings.com.au/spring-co...-c822_p3971210
Just be careful that you don't stack the spring (compress it so much that it becomes a solid coil), I guess it could be cut down by a couple of windings as well if needed.
I haven't tried these myself.