http://www.mogosmojo.com.au
they deal mostly with racecars, but if you emailed them asking where you can get die cast semi trailers they should be able to help
or you can try hobbyco, they’ve got a huge selection of diecast models http://www.google.com.au/url?q=http://www.hobbyco.com.au/product_groups/display_group.php%3Fgrp%3D7&sa=X&oi=smap&resnum=1&ct=result&cd=4&usg=__M2dRSp1KFm77Yb1x_85lq_izUH8=