I assume you are already using our SonarC# analyzer!
In this very forum you can suggest new rules (see Suggest New Features > New Rules, and quite regularly this results in us building new rules that benefit everybody using SonarQube. You don’t even have to pay our developers, although maybe somebody will slip you their Venmo. (kidding)
Of course, the SonarQube Marketplace is available on your instance in Administration > Marketplace where some plugins are available. I can only think of a few instances where people actually sell SonarQube plugins – they aren’t in our Marketplace and I can’t think of any for C#. I’d encourage you to suggest a rule through us.
Finally, if there is a rule available in an external analyzer, SonarQube is getting more and more friendly with importing reports from popular tools.