In addition to the common booking.com and Airbnb, there are many other channels that can help diversify your offer. For example, if you want to target the American market, consider opening sales on Expedia, which also offers flight+accommodation packages, historically with a very low cancellation rate.
If you want to target the business market, you could consider partnering with HRS. If your goal is to start working with travel agencies, you might want to explore the world of GDS (Global Distribution System).