idiom

I went over all the idioms listed at

http://www.goenglish.com/Idioms.asp

But I couldn't place my hand on the one I'm looking for. I need an idiom that implies that a certain group of people were "not alone" sharing the same opinion.

Two people with very similar opinions or characters could 'be cut from the same mould'. I'm not sure if this is exactly what you are looking for, but it might be close.

<<I couldn't place my hand on the one I'm looking for>>

You mean, " I couldn't *put my finger* on the one I'm looking for", actually.

<<I need an idiom that implies that a certain group of people were "not alone" sharing the same opinion.

Two people with very similar opinions or characters could 'be cut from the same mould'.>>

I think George may be mixing up "cut from the same cloth" with "come from the same mould". But both mean the same thing -- people who are very similar in outlook, or "alike as two peas in a pod".

If you are telling someone who is already like-minded your opinion or arguing a point that they already agree with, they may smile and tell you that there is no need, because you are "preaching to the choir".