Someone mentioned the PIG 2, which would work well if you wanted the post lamps to do more than just go on and off with GI.
You could also use the GI Buddy which is a cheap DIY board that would allow you to use an outside voltage source and have the GI or insert lamps control a post lamp.
http://www.dkpinball.com/DKWP/?page_id=827
Just another option, especially if you already have some all hooked up with 12v and don't want to have to re-buy everything.
Mike