Have a 1999 406 HDI 2litre turbo diesel esate. Alternator belt snapped!!

Went to fit a new belt and noticed that the alternator pulley had moved and
the belt had just been running on the shaft b4 it snapped.

On removal of the pulley from the alternator there appears to be some sort of
bearing which has disintegrated inside it.

Has anyone any idea if the pulley is available anywhere or would I need to
replace the alternator. On other makes the pulley cannot be purchased on its
own.

Thanks in advance.