HO tire ?

I am setting up my HO track again after 12 plus years. My problem is that most of the tires are no good, they are eather hard as rocks, or crumble when touched. I have several different types of chassis, with a wide variety of tires. Who sells replacements, and will be able to help me get the right tires for each car, should I get stock replaceements, or would silicon or urethane be a better choice?
Thanks
George
Thanks
George