You just have to keep a keen eye out on different sites and be quick as there not as common as pythons.. Amazing amazon have common tree's for sale and there's a bloke on this link that has commons also.. If there not what your after try posting a thread up in the wanted section, I'm sure someone can help you out or give you directions to someone that has got something your after. I had the same trouble looking for night tigers and blue phased common tree snakes but eventually snapped a pair up.
Well had a scare tonight ... I went to check gary and..... No gary. Now I know he cant get out and I pulled everything out slowly so I didnt miss him... No gary. So I slowly ran my fingers through his kritter crumble amd out pops gary... Now since when are "tree" snakes classed as burrowing snakes lol