You can buy license for Plugin  from Attlassian Market.

After you bought your license from Atlassian Market follow these steps to apply:

  1. Log in into JIRA as Administrator.
  2. Go to Administration section.You may need to log in again (we're super secure).
  3. Navigate to the Manage Plugins page of your JIRA instance (the Install section in older versions).
  4. Locate your installed  add-on in the list.
  5. Click the add-on entry and copy and paste the license key into the license box for your add-on. All done!