AC Trouble in 83 300D

[rfdillon]

Folks, a couple of weeks ago I drove home on a Monday evening, and had cold AC when I shut off the engine. The next morning I fired up the car to go to work, and no AC!
I took it to the shop, who tells me that there is a blockage somewhere in the AC system, and that the compressor is making a bunch of noise. I'm not real sure they know what they're talking about, as they don't work on many old Benzes, but does anyone have any suggestions as to where they should start? Also, if I need to purchase an expansion valve, or new AC compressor, from where would I find these parts at the best price?

Thanks so much in advance for your help!