Glyn:
I have purchased a number of things from this site -
www.performanceparts4benz.com They have a catalog that is quite helpful that gets mailed out as you make purchases. Fastlane is very very good and Phil is extremely helpful, so that is normally where I make my 1st visit when looking for parts. There is also
www.millermbz.com and I have visited them frequently because they are located right here in Charlotte NC.
BTW, I'm with azurite - I didn't know there was a plastic thermostat housing.
Also, if you get to the point where you really want to fine tune the adjustment on the IACV, Steve Brotherton has a fine article on how to do that. I was quite successful in adjusting mine when I 1st purchased my 300E. You can find the article here on this site (sorry, I can't seem to remember much of the details on how I found the article. Normally when I find these little nuggets I print them out and put them in a binder for later reference).