I have bought a ball at the tournament twice over the years.
Both times, I had the pro-shop at the nationals venue (The stadium, when in reno) pull the specs of my hand off of another ball.
If you are wanting to change a pitch or span, and not sure to what, the guys there are typically experienced pro-shop operators and will have the skills to do so.
The 2 balls I bought at nationals both fit me as if bought at my local pro-shop.
If you want to spend the money to buy balls at the tournament; I would have no reservations regarding the skills of the guys running the shops.
Depending on when you go, and how busy it is, it is possible that they don't have the ball you want in stock. especially if you are there the last week or 2 of the tournament.