Displaying Taxonomy Terms which are used by a Content Type
You should use Views. Here are the steps to do to achieve what you would like to do:


Create vocab. "Category":


Create content type "Suppliers" with a "Term reference" field to "Category" vocabulary:


Create three "Suppliers" contents in "Agriculture", "Business" and "Blabla" categories - so these 3 should be listed in your block.
Add a new view at admin/structure/views/add with these settings:


Add a new field "Content: Category":


(here are the settings)
Remove "Content: Title" field (1 , 2)
Set some descriptive names (like machine name, block name, display name, etc.), AND set Sort Criteria to "Content: Category (asc)":


Save
Go to admin/structure/block, set "Supplier Categories block" to be displayed in the "First sidebar" (or "Sidebar first" in Bartik) or any other, then hit "Save blocks":


You are ready.