The beach balls I used are linked below. The price hasn't seemed to have increased since I purchased them, they're still about $15.
Buy Intex 42" Jumbo Ball - 2-Pack: Beach Balls - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
The idiot vendors on Amazon probably have no clue how to effectively translate the sizes to the idiot customers who don't have a clue what the words "radius", "diameter" or "circumference" mean, but the vendors also don't mention ANYWHERE how they came to the measurement of 42" . Seems to be idiots all around, concerning sizes of these things once you go past a certain size, and it's open to interpretation. My suggestion is to buy the linked beach balls if you want to go that route, otherwise, buy several and make sure you can return them, and simply send back the beach balls that won't work for you.
It seems like they measured those Intex beach balls from "North pole" to "South pole", since they seem to be about 2' in diameter. If you weren't taught that term, or can't remember which one it is, the googles are your friend. You have visual proof that the linked beach balls work, otherwise you're on your own. Don't even get me started about the "Five FOOT beach ball" that I bought for the rear, it's somewhere between 3-4', and MULTIPLE people were complaining in reviews that the size was misleading, either too small or too big, but NOBODY said the measurement seemed correct. I may have gotten that from Costco or Overton's, since I can't find it in my Amazon orders.