I cannot find it in the Adobe marketplace. Does the physical boxed product no longer exist or something?
You can purchase CS6 thru the page linked below but the page is currently being fixed and the Flash CS6 link is not showing (would be in section 2)
Try this link for downloading a trial for Flash Pro CS6
It is also available for download to subscribed Creative Cloud customers.
Flash CS6 is also available on Creative Cloud.
1)Login to https://creative.adobe.com/products/flash using your Adobe ID.
2)You will find flash CC and Flash CS6 in the dropdown as show in the image below.
3) You can download either of the products. If you have a valid flash CC subsription, both products will be available to you.