I have a 1990 Si Civic and the motor mounts that are welded to the inner fender are cracked, how would I go about replacing them and where could I get the parts? Both the driver side and passenger side are cracked.

 

Unfortunately, off another car as they are not produced any longer. You may repair and reinforce the original ones as this may be easier to do, this is what I would do.
Regards
Dave