Most bolts and bearings places have good industrial grade springs and there's also companies who specialise in making custom springs from tiny things to coils suitable for freight trains.
I used to know a few companies but that was back in the early 90s but I'm sure a quick Google search will bring something up.
Are you after compression springs or extension springs?
Ah, this company is in Melbourne:
https://www.bellsprings.com.au/
They are numerous:
http://www.asisprings.com.au/default.aspx
There could be ready made springs cheaply available too in various hardware places but keep in mind there are good and bad quality spring steels (I have Chinese brass here which is extremely magnetic).