I am quite tall and always been on the lookout for longer cranks... occaisionally they show up on EPay.

I also saw Truvative 1.1 in the 180mm length for sale at one of the larger online order houses... speedgoat.com , maybe? But this is, i believe, intended for mtbs so the chainline might not work out.

If you can't find a used or budget crank in your length, most manufacturters only make them in their upper end parts (Dura Ace and Record are available in more lengths); or Profile Racing BMX cranks can be installed (again might be chainline innues), surly MR Whirly, Zinn, or whatever company makes cranks for Zinn.