Replacing a lost disc brake pinch bolt
The bolt is called pinch bolt, that's the standard name. It is a 5mm steel Allen bolt of 6 to 10mm in length, of the same type that is used with bottle cages. Alternatively a bolt with a hexagonal head may be used as a stop-gap. The loss gets a bit 'hairier' since you've lost the cable pinching plate as well. It might require some hunting for a suitable part or the need for some work to make one out of a washer.
The same type of cable pinching bolts and plates are used with standard rim brakes or with cable activated derailleurs, front and rear. Your LBS might have a bits and pieces box with a suitable replacement.
You may just want to check that the bolt doesn't protrude on the other side as to touch the body of the brake, in which case it requires some shortening!