Right.
I can see the problem you are facing.
King Arthur did not exist. Yes, I know there are all sorts of theories, but he didn't.
So neither did his knights. Therefore, there will be no coat of arms/crest associated.
And even if Arthur and his knights did exist, it was during a time that predates heraldry, and so they wouldn't have a coat of arms either.
You're on a hiding to nothing there. Quite possibly some artists and illustrators may have given some knights a coat of arms etc, but that is not 'official' and different artists may give them different details. And there's nothing to say they are right or wrong because, as I just said, they are fictional.
Truth is, there are no set items that comprise a full achievement of arms. When someone is granted arms they sit down with a herald and they create something that is relevant to the person. That goes for the actual arms themselves, any supporters (those things on the side holding the shield) and the crest (any helmet/hat will depend on set factors). So those supporters and crest can pretty much be anything and will be tailored to the applicant rather than a set list of options.
It could well be that someone with your surname has, at some point in history, been granted a coat of arms. That in no way implies that they are 'yours' as well. Unless your are hereditary gentry then even if you father had a coat of arms that would NOT mean you are entitled to use it. Unfortunately it doesn't work like that. Just because someone in history happened to have the same surname as you, and had a coat of arms, means it's no more relevant to you than it is to me. That's a false connection.