Thanks for all the replies which have given me food for thought.
Ideally I would prefer the washers to be stainless steel, or at least some non corroding material. I had discounted making the washers myself because I could not see how I could drill/punch/turn etc and not end up with burrs bigger than the material thickness. I suppose I could lap them on and oilstone but making 24 washers 0.2mm thick with a 3mm hole and an OD of 4.5mm would wear away my fingertips.
One of the members has offered me some washers that sound ideal so my search is really over.
I contacted the Pinstructure company suggested by Richard, They do have shim washers 3mm and upwards in carbon steel from 0.1mm thick so they are another possibility. Their website has all sorts of 'hard to get' products so is definitely worth looking at. Actually the website make them hard to find as well, even after you register (which you need to do just to look at pages of the catalogue).
Ian