The bolts for your rack are 5mm; they are the same size and thread pitch as your water bottle cage bolts. I apologize that with our stores closed we aren't currently able to supply you with small parts like this! You should be able to find these in stock at any bike shop that is open as Tom suggests, or you may also order them online if you prefer to save the trip.
Here is a link to some comparible bolts.
I hope this helps!