A couple of others have said Malwarebytes for a free solution. I agree, as they have a very effective free version, but it doesn't run in real time like a licensed version of it. It's better than nothing, but you'd be relying on your users to actively scan for threats. Windows Defender, especially if you have 365 licenses, is more than competent, and then if you'd really prefer other options, there are free antiviruses like Avast!, Bitdefender, Kaspersky, etc. They're all fine, but Windows Defender would still probably be your best option for an all in one solution.
And if you're in a situation where the business doesn't see the value in purchasing licenses for additional antimalware/antivirus support, you'll want to explain why it is important. Try to quantify how long it would take you to resolve a virus that is spreading through the network and how much of a business, and monetary, impact that would take. If you can make the cost of getting licenses for all users cheaper than the cost of risking computer downtime that affects revenue, they should be more open to following your advice.