Have a look at www.moto-racing.co.uk.
You want to follow :-PERFORMANCE PARTS, Chains/Sprockets, Blank & custom sprockets, YOUR SPECIFICATION - 520 sprockets.
They make to order £33.45 + £7.05 for gold anodizing + about £14 delivery to Europe.
You have to tell them the bore, bolt hole diameter and PCD.
Now here is the bad news if the price has not already put you off.
The MC21 sprocket has special studs which you would have to remove from your existing sprocket.
These are a tight fit so you would need to accurately measure the diameter in sprocket and convey this to the manufacturer.
I also think the side is recessed for the carrier. There might be extra charges for these.
You might want to look at www.jtsprockets.com and search the online catalogue for NSR250 for the dimensions.
(A 5 hole PCD is difficult to measure).