Easy to do very hard to find the mirror unit second hand, unless you're prepared to pay for a new one.
One option is to go for an E class mirror which are more common but a different shape. I recall Seanhump has done this, a search should help.
Or consider a non-merc mirrors, Ford & BMW seem most common. Some of the Z series BMW mirrors have the same oval shape as the SLK, but many of the BMW mirrors have an aircon sensor on them which would be odd for people who knew the small red dome wasn't a Merc feature. The rest of the world wouldn't notice.