I have a 77 450SL as well and am looking for similar parts - front spoiler and left/right fenders. Have only found two places that sell these :
http://www.performanceproducts4benz.com/ and www.adsitco.com .

Performance puts items on sale (I missed their 20% off Halloween sale) so will probably wait to they go on sale again. Adsitco give 10% off on web orders but don't have as many other sales.

Good luck.

77450slowner