I would check with your local place as they might have several different sizes you can borrow from during your class/rental time! Especially if you're going with a puppy who will outgrow stuff very quickly. They may also have different brands so you can try before you buy.
The place we go, we just use one of theirs. We don't have our own as we've not needed them outside of class/pool rentals (well, the one time we did 'need' one, our trainer actually let us borrow it for the weekend because she's the bee's knees).
Both my dumb dumbs with their borrowed jackets and you can see in the background on the left, there is something in a size for everyone!
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This was a different pool rental and they also had vests to borrow during our pool time
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