The perspex windscreen on my flybridge is screwed on with a cup washer on one side of the Perspex and a shoulder washer on the other, as shown in the photo attached. The cup washer is solid..
Do you know where I can source similar fittings?
Depends on how exact. Ebay as you would expect has plenty of white plastic cup washers and stand offs. There are some specialist suppliers as well but that will take a little more research. I find things like this can almost always be found using Google, but it may take an hour of research! I was missing an insert for my rails and found one company in China that sold exactly the right part, next to nothing for a bag of 100, alebut I only wanted one. The bag of 100 was 1/20 the price of one from the boat manufacturer, never mind whether they had it in stock and the postage from USA which was almost as much again. Post from China - free, adn even the two weeks wasnt too bad.