Is there a list of US (United States) only bridges for tor? I need to relay thru US bridges only.
As far as I know there is no such list and it is not possible to get such a list. The idea of bridges is that they are hidden. So you won't find a public list of all bridges or only a greater subset of all bridges.
In general you can use any bridge from any country. Just go to bridges.torproject.org, solve the CAPTCHA and receive your bridges. Or go to about:preferences#tor in Tor Browser and select a bridge.