I bought some very similar headless screws a year back for a MacBook or MacBook Pro. From memory they were very close the the two Mac mini ones but stainless steel.
See for example
http://www.ebay.com/itm/New-MacBook...Drive-Screw-HDD-Screws-4pcs-Set-/290691184348
I have a feeling they could be just a hint smaller?
They are readily available.
Some one might be able to confirm if they are similar size.
If you did not have a HDD in the lower bay under the wifi grill, did they have the screws that screw the HDD to the wifi grill? If so what did they screw into? Sounds a bit strange as the screws create a nice flat seal across the grill for the black base (on the inside)
You may need these screws depending on how they set your mini up.
http://www.thebookyard.com/product.php?products_id=7399
If you use conventional HDD screws to screw it to the wifi grill, you will either need the above screws or some what are very flat because the black base seals across the grill and screws. The black base is also likely to catch on conventional screws as well.
Final edit:
Found this. It seems they are called "mac mini lower bay alignment screws" in a search
http://www.impactcomputers.com/922-9584.html