kinetico model 60 water softener

I have a Kinetico model 60 that is about 30 years old. I occasionally end up with hard water, even though the system is working.I had a Kinetico rep come out and he tested everything, took the tanks apart and said the resin was shot. He wants $600 to change resin in both tanks. Can anyone tell me how to do this?where to buy resin?Any other info would be much appreciated.