www.makemkv.com

how do i go about buying a key for this program?

Currently, you cannot. The link to the "buy" won't do it, and the author has said as much. Currently, you can only use the beta key, which expires in a few hours (unless Mike gets here and renews it).

Until it's re-issued, you are limited to setting your computer date to something before the end of the key period, starting MakeMKV, then resetting your computer date to the correct date. That will bypass the checks for as long as MakeMKV is running.