Front arm plastic got cracked. How come I can only find rear arm cover replacement? I don't need the motor and cabels, just the plastic.

Probably because you haven't performed the correct search or looked in the right places. I found both with a google for "DJI Mini 3 pro replacement front arms".
BTW are you aware that any NEW parts are probably going to be pattern parts? As far as I know DJI do not make such parts available to the general public.
Besides it may be tricky to swap over the motor, antenna/e and wiring.
If you end up getting an arm and motor ensure that the motor is for a Pro. The non pro motors lack the blade mounting boss of the pro motors, they may be interchangeable but you would need the different screws used with the non pro drones.

Hi there. Thank you for reaching out. We're sorry to know the incident that happened on the unit. In this case, I'm afraid that you may consider sending the unit in for a proper diagnosis. You can contact our support team to start up a ticket at https://www.dji.com/support . Thank you for your understanding and support.