I was able to pick some up at a local auto parts store. Any brand will be better than what comes standard.
Having said that, my advice would be don't waste your money. My experience is that trying to turn the music up loud enough so that you can hear it over the tractor is a good way to accelerate hearing loss.
Instead, I gave up and bought a decent pair of bluetooth noise cancelling headphones. Now I can listen in relative silence/comfort and not worry about further damaging my hearing.