You can if they have them, but they are usually regular issue. First and last prefix, RADARs, solid block serials etc as well as crisp uncirculated regular notes are sold by dealers. Since the RBA stopped selling to the public, Downies had the rights to sell these notes, which I believe they had to tender for.
Generally I get most of my notes straight from the bank or ATMs, but to get first and last prefix, you don't have much choice but to get them from a dealer.