Is the 8 circuit breakers holder part that is attached to the center of the breakers metal box by two screws could be bought separately? If yes where can I buy it? The metal electrical circuit breaker box size is 8" W X 11" H and has 8 spaces for 8 circuit breakers. Thanks for your answer.

Probably not. If it is older than a few years almost definitely not.
If it is a viable part number you would be much better off just buying a new panel and replacing the guts. When all is said and done I bet there is no cost difference and you'll be done probably weeks quicker. NO ONE will stock just the guts. If it is even available it will have to be special ordered.