Such a broad open ended question . . .
There are different sources for performance upgrades, appearance upgrades, wheels and tires, in car electronics . . . I don't think one place has a lock on the best deals in all categories.
Hard to find OEM pieces are probably best hunted down on German E-Bay. Just saw a new, old stock, AMG ducktail spoiler for the 124, totally unavailable here in the US, but it sold for under $100.