I had this very issue a while back. As folk know, I'm a great believer in using whats been thrown. You can use the side out of an old but sturdy plastic water container as its polythene and other than being white, almost identical to whats there. Even a new water container doesn't cost that much and its not hard to pick up a container like that, no longer wanted. I see them all the time. Just cut to shape. Will outlive you probably. Use some dividers if you want to be precise - I found a food plate that was about right size and cut round that then made a smaller hole for centre.
Other things that 'spring' to mind are £1 shop hot water bottles. Pair of scissors and you can make two rather good spring seats, plus you have a choice of colours too - red, blue, yellow, green.....
For goodness sake don't bother ordering one. Just not worth it. My rear springs sit on a former plastic water tank.
They're just there to reduce friction and stop any rattles.
