How I can get Huaihua from Wuhan by bus and where can I take that bus? Thank you very much.

I often take the train, seldom taking the buses, because I cannot find bus information online.
I also suggest you to take the train and the train takes about 10 to 12 hours. You can book a hard sleeper on a overnight train, so that you can sleep on the train at night and reach Huaihua on the next morning.
For the train detail information, it is suggested to look at the Train Section on this webstie.