Hi,
I had the same problem before, I took the tumbler out of the car, and I took it apart, it is not sealed, it can be serviced, but in my case one of the chips were not Ok, I straightened it and filed, and put everything back with good grease..everyhting works fine...I do not believe you could buy these chips (slices) from a dealer, but you may try locksmith, or from a used lock etc., they have numbers on them, or if you are lucky you do not need. I realized that there is a week point in these tumblers that can break and make them hard to turn.
If you still in need of more instructions, i can provide more,
in my car 1987 300D, the price of new tumbler: $355

goog luck