There is an aluminium one for sale on eBay https://www.ebay.co.uk/itm/132917359815?ul_noapp=true it will not be cheap but I know there is an Essex member who is looking for one.
Best wishes Chris
Some serious bidding going on there.
May be worth watching to see how much it fetches.
As one of my previous bosses used to say “Where else will you get one and how much will it cost?” 
Steve
I bought a new replacement one in the 90’s from Motomecca and it was about £30.00, now they are almost impossible to find, but I have seen a plastic one of the same shape.
£92 !!!
Makes me think about breaking my bike!
There is this on eBay, you should ring them and see if it is plastic or aluminium. A lot cheaper than the one just seen above on the same site…
Just got a reply from Nik W the one he has for sale is plastic, I guess the price made that evident