Welcome to the Community, @Tameliathe.

To purchase QuickBooks Desktop for Mac 2021, you can contact the sales department to assist your preferences for the product further; you can access this using this article: Contact QuickBooks Desktop support.

Here's a photo for your reference:

Note that you need to ensure that it is updated to the latest release to be compatible with Sonoma.

After purchasing the product, you can find instructions on downloading, installing, and activating QuickBooks Desktop in the Downloads & Updates section. 

Feel free to reach out if you need further assistance or questions about QuickBooks Desktop. I'll be glad to assist!