You could try plumbers merchants, I used alot of tap washers to bed a windscreen on a while back. I found I could get all sizes, rubber fibre and plastic.
While I am sure that plastic washers are available from somewhere, no one seems to be coming up with the secret answer, and I would not know for the UK anyway.
But I have done the same as Davy_S. A prowl around a plumber's merchant that supplies to the trade, including the manufacturer's spares for standard plumbing fittings often turns up something either suitable or modifiable.
For example plastic tap washers (and other similar bits) as washers (as has been said) and many of the plastic/rubber gaskets that fit in hose fittings or under flanges work ok for isolation or for deck through fittings eg for a deck washdown connection or anti-vibration mount for a small pump. I just make sure they are not used where exposed to the sun or oil.
While I have never looked for washers in such a place, another useful place for a browse along similar lines when fitting a boat out is a hydraulic hose supplier/maker (colloquially "hose doctors" here) - have lots of interesting little bits and pieces and often know where other bits can be obtained from too.
Noticed in the ASAP Catalogue they sell plastic washers in packs for use in fixing
sound proofing.
On the same page as the sound proofing material, dunno what sizes they were though !