I don't have any direct knowledge at all, but what I do know about getting vaccinated in the US is that it's a bit of a free-for-all, regarding getting booked in (walk-ins are welcome in a lot of places, now), and there doesn't seem to be much tracking of who's been vaccinated, or with which vaccine. A lot of sites don't say which vaccine they're giving out, but some online groups will track that, so you can try to find out where Moderna is being given and go there. I would think you'd have little difficulty with getting your second jab. But, again, I have no direct knowledge.