Energy suspension have 7/8" universal bushings. See if one of those might work. A problem with polyurethane is that it really needs to be greasable otherwise, it will start to bind and could make the ride more rough. Since the body is rubber mounted, you may not ever notice it but that does not change that it will happen.
If it is only that one that you're having trouble with, you could try having an aluminum, bronze or maybe even delrin bushing made. The spring would still have a rubber bushing so probably no problem with that.
Another thought is to try Eaton Detroit Spring. They claim that their springs are made in the USA. For the price, typically at least double any other spring, it seems silly that they would cheap out on the bushings so maybe they have something good to offer.