My clear retainer have a crack but it's not not completely broken should I buy new ones or do they still work?

It depends on where the crack is located and how large the crack is. I have seen small cracks at the very back of the retainers that I was able to remove for the patient. This was a simple fix to allow the patient to continue using the same retainer. I have also seen cracks between the teeth which compromise the fit and effectiveness of the retainer. Keep in mind that the lifespan for these clear retainer materials is 16-24 months. So, if your retainer is older than 2 years, it would be best to purchase a new one from your orthodontist.