What do you think it's the best way to find beta testers for a brand-new B2B SaaS?

I partnered with an agency that's doing cold outreach. I decided to create the cold email sequence on my own, as it was easier and faster than sharing the idea with the agency, but they were able to provide me with lists of thousands of prospects matching my criteria and handle the entire campaign delivery. With that, I managed to book around 25 demo calls within just 2 weeks with the target audience to gather their feedback, look at our product through their eyes and understand their pain points. It wasn't sustainable, but was a good start to get some eyes at the product.