For McLeod, you can purchase on-site. Best option is to call in advance for plantations you are interested in and get details. Some larger plantations you can buy tickets in advance.
You have to get them on site
We bought ours on site as well. The plantation tours are non refundable, so we did not buy them ahead of time. McLeod was not very crowded and is close to downtown.
Thanks! Your answer is awaiting moderation.