Will it be hard to find parts for a discontinued car in the USA?

For routine maintenance it likely won't be an issue for as long as the vehicle lasts as those parts will still be made as they fit other vehicles. Where it could be difficult is if you have a major accident or if some odd part fails that is considered a "dealer only" item or some strange option item. However those things can be a problem anytime not just in the future. The same situation has happened a few times over the years as some brands were shut down like Oldsmobile, Pontiac, Saturn, Mercury, Plymouth, Geo, and models are dropped or replaced with different designs. Usually parts are still around for the popular ones for quite a while, then there is always the used parts option as well. In short I wouldn't worry all that much about it being a problem for a while.