Is it more expensive? Where do you search for these flights?

In my case it was not more expensive, no. But I was flying in Europe. Not sure about the US.

They just use them for the quieter flights where they have trouble filling up a 737.

I was dreading it because the last time I flew on an airliner this size it was a 717 and it was horrible. Very noisy in particular (I was seated right beside the engine I have to say). The Embraer was really delightful and it has under-wing engines.

Here in the US basically all the regional airlines use them. Horizon Airlines is one that I know of specifically, and Wikipedia lists Sky West, Envoy Air, Mesa Airlines, Republic Airways, JetBlue, etc, etc. Many of them contract for the more well–known airlines like Delta, US Airways, and so on, so you could end up on an Embraer even if you don’t seek it out.

A lot of those are the smaller and older regional jets like

https://en.wikipedia.org/wiki/Embraer_ERJ_family

which personally I like less than

https://en.wikipedia.org/wiki/De_Havilland_Canada_Dash_8

The newer and larger E-Jets are on a whole different level in terms of comfort.

Delta has some A220s, I got put on an E-Jet on an American from Philadelphia to Montreal.