Techron fuel injector cleaner is sold in many auto parts stores, even the low-rent chain ones.
You almost certainly have bad gas in there, and it will take a while to get the plugs clear once they get sooty. If they are Bosch, get the fuel system clean by running a fresh tank though with Techron in it, then replace the plugs. They recover from carbon fouling VERY slowly (except the Platinums).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!