Hi Aaron
The lead should of come with the speedo when you got it unless you bought second hand, you should be able to get it from Rc disco as there the uk importer for tekin. have a look at the link so you can see what your after.
http://www.amain.com/tekin-gen2-hotwire-adapter/p255553
not Rc disco but i just found it quicker on amain site.
Sean.