Try your local Screwfix or Toolstation. You'll need to check the size you require, so some careful thinking and measuring would be in order. It might be an idea to go for stainless steel screws.
You could always seek outcIgnis customer service and drop them a pleading email asking for the size you need and maybe asking if spares are available and what the part number is for a set. You might be able to wangle a set as a freebie - you never know.
We may have found other answers which could help you.