Hey Andrew,
The screws alone are not orderable as spares from us, part WS-X6K-SLOT-CVR contains the blanking plate and screws.
I believe the screws you need use a standard 6-32 thread, and there are several accessory shops which can provide suitable replacements - try "WS-X6K-SLOT-CVR spare screws" in Google.
Hope this helps!
/Phil