EBO correct I have only seen them a few of times once in Alaska, Yellow Stone Park and in Maine at a friend’s house.   I would figure the Northeast is the best chance.  Although they are also in the Northwest.  The problem being in the Northwest the country is so rugged that it would be hard to find them in an not protected area and you also have to be in an area that has good water run off all year long.  That leaves almost all of California out of the picture.   Hunted a lot of Northern California when I was younger and never saw a beaver dam.  

Just finding them is only about 1/4 of the problem.   Seeing them work it is a bigger issue.  If you really want to see them it will take a lot of patience and luck but the Northeast or Yellow Stone would be your best bet.   

I actually never saw one outside of a zoo. I just saw active dam's they were maintaining along with their houses.  They are not much for being seen.  May be it was the fur hat thing their daddies told them about. 

My friend in Maine had on hell of a time with them for a while as they dammed up the stream running through his land and flooded about 1/3 of his field.  They were hard to convince to leave he said.  

If you want to help yourself in that area, make a little hole in their dam and then get back out of sight.  They will come up and fix it when the water level starts to drop.  Left on their own they may not come out during the day at all. The hole in the dam thing is what they used to film a lot of those nature videos.  It does get them out working in the daylight.  (According to my friend in Maine.  They finally left after they got tired of rebuilding the dam.  Took him multiple dam busting trips to get it all done.)