At this moment, Firefox OS is not sold in devices targeted for the USA. You can however find devices for sale on sites such as eBay from verified dealers or sellers. Please note we assume no liability for purchases made on 3rd party sites. You can view a list of devices here: https://www.mozilla.org/en-US/firefox/os/devices/

Didn't answer the questions of 1)when, and 2)is it possible to install o In place of android on my existing phone

There is no expected date, sign up for the newsletter and keep your ears wide open, you may find devices sold strictly online which can ship to the US.

You asked if you will be able to replace the OS, not install it in place of Android, those are 2 separate things. You can learn more about installing Firefox on your device here: https://developer.mozilla.org/Firefox_OS