A few suggestions for you:
Trident – though they seem to have 19mm stocks and I suspect 21mm is normal today.
Ian Porter Sales http://www.porters.org.uk/spare%20parts.htm
Pinnell and Bax – the modern metal affairs.
http://www.hartleylaminates.co.uk/ – ditto
The commercial offerings are not cheap. If you have the time and skills making your own is a good option. There is an official drawing for the rudder blade – if you wanted your boat to be measured the rudder blade should be made to drawing.
There is a tremendous strain on the rudder – so best quality marine ply is minimum – good service from;
and please read various articles on WIC;
hope that helps