It depends on the details of your project. Unless a proprietary package meets your needs spot on, I think you’ll find that customization is needed at some hourly rate. You would have to talk with their sales reps to determine cost and availability.

Same for open source. It’s just that developers may or may not be available who can meet your special need. Example: I was hired in the past to upgrade sendmail which had been customized to do auditing. And it worked well.

As for availability, you could ask a consulting outfit.