Lesser known companies that we use in our field

So some of my best investments have been companies that are household names in my field, but that the average person has never heard of. For instance, im in the trades (hvac), and one of my best investments ever is AAON. My sister is a dialysis technician and she told me about fresinius, which is up like 35% in the past 3 months. What are some niche companies in your field that most people would never hear of to even be able to invest in?