hi,
you need to use fetch xml and javascript.
You can find a solution here:
http://muhammetatalay.blogspot.ch/2013/02/how-to-filter-lookup-in-crm-2011-by.html
http://www.powerobjects.com/blog/2013/08/05/creating-custom-filtered-lookup-crm-2011/
http://crmallied.blogspot.ch/2012/04/filter-lookup-values-using-javascript.html
bye
Alessandro
Alessandro Graps