Where can I get a replacement tension knob and anything that might have been inside the knob? My son received the combo as a gift and the knob was missing.

 All Abu directly:

 

800-228-4272

 

 

There is no option for parts per ABU Garcia

I have seen many combos on the racks at Dicks and Walmart with missing tension knobs. Consumers should examine any store bought rods and reels not in clamshell packs for damage and missing parts.

 

in this case you say it was a gift. Hopefully it came with a gift receipt. If it didn’t you can hope the store will do an even exchange without a receipt. 
 

The unethical thing some folks would do is go to the store and take a cap from a combo off the shelf. That leaves the next person with the problem you’re experiencing…