How do I get the cap on the cold water knob replaced? Half of the pieces underneath are broke off and the cap keeps popping off

These little caps are not very strong and are very easy to break the little prongs off. I think the only way to find replacements are to find a plumbing company close that has a shop, stop by there and see if they have some old ones that they have saved from replacing the complete faucets. Otherwise you will have to go to someplace like Home Depot and find handles that will fit and exchange out the old handles.