You can buy XenForo from the official website of XenForo. Here are the steps to purchase XenForo:
1. Go to XenForo's website at
https://xenforo.com/
2. Click on the "Purchase" button in the top right corner of the page.
3. Choose the version of XenForo you want to purchase and click on "Add to Cart."
4. Review your order and click on "Proceed to Checkout."
5. Provide your billing information and select your preferred payment method.
6. Review your order one last time and click on "Place Order."
Once you complete these steps, you will receive an email confirmation of your purchase with instructions on how to download and install XenForo.