What you need is a table for the items to be selected in the second value list. Each record (itemsupply) will have a category assigned. The first value list will be based on the category field so it will be dynamic - new categories will appear when you enter new categories in the table.
The second value list will be based on a relationship from your inventory table to your itemsupply table that links the category fields. This value list will show only the items that match via the relationship - that is those items that have the same category of that selected.
Difficult to explain and if I could post a sample file I would ;) Post again if you need further help setting this up.