Best Methods to Improve Contact Coverage for Mid-Sized HVAC Companies

Hey all, does anyone have advice on how to get better coverage on all of the contacts at a company? I have a table of target HVAC companies between 100 - 1000 employees. The key people I'm looking for are Directors of L&D or Operations and the CEO or COO. I find that Clay's 1 credit "Find Contacts at Company" enrichment gets me better results than the free "Find People". I tried Surfe's Find people enrichment but the coverage appears worse. I'm also using Open Mart but it's a bit expensive - so I have logic to only run it if no one else could be found. Any other data sources or techniques you recommend for finding people at these companies in this 100 - 1000 range?