You can purchase WindowBlinds here https://www.stardock.com/
Once the purchase is completed, Stardock will send you an e-mail with a serial number and download link for WindowBlinds.
Note - you can also download "Stardock Central" and install it after receiving the e-mail. When you launch Stardock Central for the first time, it will contact the server(s), aquire your account information, and automatically offer you the full version of WindowBlinds. The only possible issue with this method is that your account information may take more time to update, and may prevent Stardock Central from receiving said updated information for a day or so.