Hah, curiously, I went searching for various coils. Didn't find that but found something called SuperSprings SumoSprings. It's a foam rubber insert that fit inbetween the coils. Super easy to install too, and they have various applications for tons of vehicles. They seem well reviewed too!
video - SuperSprings SumoSprings Coil Spring Stiffeners
Pretty well reviewed on Amazon - https://www.amazon.com/SuperSprings...F8&qid=1507431714&sr=8-7&keywords=SUMOSPRINGS
video - SuperSprings SumoSprings Coil Spring Stiffeners
Pretty well reviewed on Amazon - https://www.amazon.com/SuperSprings...F8&qid=1507431714&sr=8-7&keywords=SUMOSPRINGS