I'm shopping for a pair myself. I also wonder about quality. There is a seller on eBay selling mounts made by "Hamburg Teknik" in Germany at a steep discount to the OEM price. I think I may try him.
And I looked at the S500 mounts on Fastlane out of curiosity. The OEM mounts on my SL are by Boge -- the cheap ones. The expensive ones are by Lemforder. Steve Brotherton says they are a good brand and specifically recommends them for suspension components