Someone posted a while ago about finding cavy rescues in WA:
Home - Furry Critters Halfway House
I adopted a little boar from here recently.
Saving Perth Animals From Euthenasia
Have been in touch with them as well and they seem very knowledgeable and dedicated.
Obviously RSPCA too.
Gumtree is an option but beware the breeder ads. The two rescues I mentioned are not involved in breeding but do get pregnant cavies and rabbits surrendered to them.