Where do the electrons come from to make the carbon stable during bombardment of alpha particles on beryllium
When beryllium is bombarded with alpha particles, where do the electrons come from to make the carbon stable?


Nowhere.  This is a nuclear reaction, not a chemical one.  Charge is conserved and no extra electrons were supplied.

The carbon nucleus is stable with respect to nuclear decay.

Chemical stability relates only to the behavior of electrons with respect to atoms.  It has no place in this nuclear  reaction.

Eventually the carbon nucleus may attract additional electrons from some external source, but that's nothing to do with stability of the nucleus.  The nucleus does not require any electrons to be stable.