Many shops will install shorter, stiffer springs....which are available on the internet. I don't know how they compare to 500E springs, but I'm sure it will be an improvement.
In fact, I believe PartsShop link on this site shows an entire suspension mod that emulates the early 90s "SportLine" suspension for the 300E, 300CE, 300TE.
p.s. My 300TE rolls, but in single curves, I just add 20mph to the posted limit, and let it hang out. My wagon still kicks the snot out of any SUV around a curve. Truth, baby. 200k and 28mpg.