www.mercedesdismantlers.com is certainly one of the more popular yards AND I would call it a junk yard. It isn't very well organized.
However the prices are fair. The inventory is pretty good. Mark is the owner and he or Peter are helpful. They also rebuild transmissions in house HOWEVER after seeing their shop and visiting Transmeister in Orange CA, there is NO comparison. Transmeister runs almost a clean room compared to Sun Valley Mercedes Dismantlers.
AMC Auto Salvage in Wilmington is more organized, however their inventory of good 123 parts is pretty tattered. The cars are unprotected and most have been picked over pretty well. Prices are generally higher than SVMD.
Neither place has a good inventory of GOOD interior parts.