Do you have an Ohlins rear shock, or a Sachs?
Depending on the shock brand, the spring's diameter may be different.
However, there are springs available from different sources depending on your place on the planet. Are you in the US? or Europe?
https://www.motocrosscenter.com/shop/en/ seems to have good stock of parts if you are in Europe.
In the US there are also several good suppliers. They have been listed in several recent threads.
I think a dealership should be able to get you what you need, but many will have to order it in.