You can buy a license key through the official website, but the problem is the purchase page appears to be broken which means that purchasing is not available at this time. However, you have the possibility to contact the developers using the following online form: http://www.pcmri.com/contact-us/
On a side note, I would recommend staying away from applications of this type, because there were many cases when users where scammed to install a non-working product. Use an official product, like Malwarebytes.
