If you already have an existing product, a good place to start is your existing customers who are already familiar with your brand and products. You can reach out to them via email, or via in-app messaging.
If you don’t have any existing customers yet, you can find beta testers via online forums and communities or by creating social media ads calling for people to sign up for your beta testing program. If you’re having trouble finding beta testers, a good idea is to offer incentives, like early access to the product, discounts, or even free products.