Populate one dropdown from another

I know this should be simple, but it has me stuck.  I've added some new columns to Places and I want to populate dropdown "B" (a list of regions) based on a selection made in dropdown "A"  (a list of states/provinces).  Has anybody made a tweak like this to Places? 

I know this should be simple, but it has me stuck.  I've added some new columns to Places and I want to populate dropdown "B" (a list of regions) based on a selection made in dropdown "A"  (a list of states/provinces).  Has anybody made a tweak like this to Places? 

You may want to have a look at these:

http://www.boonex.com/m/Dependent_Fields

http://www.boonex.com/m/Self_Manageable_Dependent_Fields

I use the Self Manageable and it's great.

Another 70$  that too for a dropdown? this should have be an existing functionality...

 But will these work with a 3rd party mod such as Kolimarfey Places?

According to Anton, that tool works only with Profile Fields