Nintex Newbie

Unique Drop-Down List Help

I have two lists, Main and SourceData.  SourceData contains a column named SerialNum with list of unique items (serial numbers in this example).  Main contains multiple columns, one of which is a lookup column connected to SourceData's SerialNum column. 
Here's my requirement:  I need to show only items that are unique among BOTH lists in the user drop-down selection box on the Nintex form (i.e. remove items from the drop-down that are already in the list named Main).  Second, I would prefer to do this using Nintex forms and javascript (if required) vs a workflow. Thoughts?
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