Rob, you might be taking it a bit too far with the "all original", but just in case you're fully committed to it:
Step 1: Find the NAS shop manual/parts catalogue. Here's a link to the parts catalogue page with the side-mount repeater you're looking for:
Step 2: In the parts catalogue, look for the specific P/N for your screw (sorry, don't want to download the manual above, as I have a ROW truck) and then order it through LR dealer, Rovers North, LRDirect, Ebay or whatever.
Alternatively, you can sometimes tell the bolt/screw type and size from the Land Rover P/N, so you can just get an equivalent from a decent hardware/fasteners store. Unless it's stainless, though, it will look like crap eventually, so, you might want to reconsider your "all original" approach,
------ Follow up post added September 26th, 2017 02:23 PM ------
Here's a link to complete NAS parts catalogue:
Index of /neillt/d90-manuals/Parts Catalog