Or suggest them to your employer. If you hear of a job you think might be perfect for one of your former bosses call them up and tell them about the position. Even if they are not interested they will appreciate your keeping them in mind. You worked for someone and got canned. Put them on your reference list anyway.
Most employers are suspicious when all the references speak in glowing terms.
During the course of a career most of us have worked for someone we didn't like or couldn't get along with. Tell them you are not sure what kind of a reference the person will give you. Most prospective employers appreciate an applicants willingness to give a reference that may not be complimentary. We also find that most people who have had to fire someone bend over backwards to give the person a fair reference. Most applicants are hesitant to call up bosses that have fired them.