looking for my family kamon (Yokono)

My great grand parents came from Nigata prefecture.

Good luck!

Only samurai families have kamon. I do not believe Yokono is a samura name.

Sometimes non-samurai families do have kamon, but those were made up by themselves after the Edo period.

I just did a simple google search which involved searching for "横野" (Yokono)* and "家紋" (kamon) and didn't get any good results at all, so the above poster is probably right and there isn't an official one for your family. Doesn't mean you can't use another! To contrast, searching for my name + kamon in google images gets me picture after picture of our mon, so...

*These kanji are the usual ones for this name, but your kanji could be different.

I don't know that Kamon but I found the place name "Yokono" in Nigata.
https://goo.gl/maps/dDj4M

By the way, in this case, I can't agree with Nihonjin.
Common people in the Edo period wore Montsuki (means "Kimono with Kamon") as their best clothes.
And even if it made up after the Edo period, it's correct Kamon today.