I didn't bother calling any of my local orchards in the end, as I stumbled across a new source for my wood chips from somewhere right under my nose!
There's a business down the road that sells supplies for landscaping, pergolas etc which I had never bothered calling as they only deal in softwoods. Purely by chance I was at the nursery next door buying some plants for the garden when I noticed a huge pile of firewood in the yard next door, so thought I would check it out.
When I walked through the front door the first thing I saw was bags and bags of American Oak wood chips and sawdust for sale. Turns out a local Cooper came to them and asked if they had any market for all the sawdust and offcuts he gets after refurbishing oak wine barrels. At $5 for an 11lb bag of chips (compared to $8 for just over 1lb that I had been paying previously) I bought what they had and am now well stocked.
I also asked about their firewood, which turns out is Australian Red Gum. Gum trees are typically quite resinous but they assured me that Red Gum dries/seasons really well, and their customers typically use it in their slow combustion heaters and pizza ovens.
I'm going to try out the Red Gum too when I get the chance, and I'll fill you all in on how it rates when I do my next smoke.