How bad is the arm you have broken? could it be welded or used and a template for someone to fab you one up?
Also i am not 100% sure, but the mustang and a few other cars stateside use the T5 box as well. the part i am unsure about is the bell housing differnces, but the boxes are defo t5's. so it might be worth looking to see if one from another model will work.
a quick look on ebay showed up this:
http://cgi.ebay.com/ebaymotors/FORD-...Q5fAccessories