Howdy!
If you head to the Buy Now page at
https://corona-renderer.com/pricing then you will find at the bottom of the page the "Add more render power." section with a link - you'll end up at the Render Node purchase page at
https://corona-renderer.com/prices-licensing/render-nodes Depending on the size of your in-house render farm, the pricing can actually be even less than I mentioned on a per-node amount, as the Nodes can be purchased in bundles. "The more you buy, the more you save...." hmm I have heard that somewhere before (for those who have watched the NVIDIA keynote this year lol).
Hope this helps!