Googling "Australian native seeds" brings up a wide variety of sites, but I couldn't tell you much about any of them. I can't seem to find the one company that I had experience with, 20 years ago. :/

http://www.chilternseeds.co.uk/item_178b_banksia_occidentalis_seeds
You can email them if you want to know if they can supply you. info@chilternseeds.co.uk

www.rareplants.de has a very large selection of Banksia [33 types] and I found them to be very reliable and fast.