Where can I find parts?

I have a Vaillant F 75 series oil fired boiler. I have no idea how old it is. I was told by a mickey mouse plumber I need a coil due to no hot water pressure. The boiler heats the water fine and heats the home fine but there is hardly any pressure in the hot water. The plumber ran an acid based cleaner through it. Unfortunately, there wasn't any differance to the pressure. Where can I find parts for the boiler and can I replace it myself?