The most obvious answer is SalesForce's app exchange. Most CRM platforms have similar marketplaces so even if you don't integrate with SalesForce those would be a great starting point.
After the marketplaces I would look to partner with sales consulting firms. There are a lot of sales operations firms and individuals out there, develop a partnership system where these firms a trained on your plugin and incentivized to sell the plugin to their clients.
Happy to chat if you want to go deeper on any of these points.
- mike
