Your question has a double take in it. You ask initially "Where in Sydney can you find Bonsai?", the answer to that is at a Bonsai nursery. Have you been to Ray Nesci Bonsai nursery at Dural? 26 Sagars Rd.
You mention driving to "find" stock, unless it is on "no mans land" which simply does not exist anymore, it is illegal to remove plant materials. So in that effect, I would be looking for areas in which you can grab the plant and get out of there.
Myself, I am more partial to Yamadori at nursery's, not Bonsai nursery's. Is essence yamadori is the search for suitable potential Bonsai material, that has been aged by nature. You don't necessarilly need to search in the bush to find this. Many of Sydney's nursery's are managed and tended to by in-experienced staff. They forget to water, knock over plants, let them get diseased, break branches off / leave dead ones, forget about stock and let them go feral in back tiny over grown corners of there yards and in general treat them with little care. The results of this are an almost perfect imitation of what would naturally occur in a harsh environment.
You can still get the thrill of the hunt from a Nursery, even more so than the wild sometimes. It presents its own version of inpenetrable terrains and obsticles to overcome, to recieve the reward of a plant. My thrill comes from trying to negotiate my way into the secret hiddens corners, where customers are not allowed, "out the back" so to speak. You want to get to know the owner, to see all their deep dark disgusting looking non sale plants. This is where the true potensai are located.
If you do locate a natural resource, send me a PM. I'm in Sydney too and am also willing to drive 3-4 hrs to me up with you for a wild stock hunt.

Regards, Mitchell.
"It is one thing to shape a tree into form, but when you are able to convincingly deceive ones perception of reality, something much more is accomplished than just a simple bonsai."
"In a perfect world, we would all be giants and all plants Bonsai."
"Grow big, finish small."
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