the only aftermarket replacement strut mounts i know of are the KYB ones, i am unsure if those are polyurethane or not. some people say they make more "bump-thump" noises than the stock ones, others have the noise cured.
as far as polyurethane, you can grease them yourself with marine grease which you can buy at like wal-mart for $5 a huge can. that's what i use on my polyurethane sway bar bushings and they only have to be regreased about twice a year before they start squeaking.
my strut mounts make noise too, but only when the car is cold...once the car warms up and the rubber gets warm it expands and i don't hear that thumping noise anymore.
i know there's been lots of discussion on this topic & its very frustrating to a lot of people.
good luck with whatever you do though
Keri