A plumber came to fix our shower and took the shower drain cover with him. He never return and I can't get a hold of him.

Anyone know where I can purchase a 4-7/8" brass shower drain cover with 3 hold down screws?

Take careful measurements and go to your nearest plumbing supply house.
You could also use the internet to find a particular style as there is a greater range.

Odd that he took it, are you sure he did not put it on a shelf somewhere?