Jerry,
As Flem points out, the last thing you want to add is water. Depending on the wine you are making, it could be expensive to top off with a like kind, such as Amarone. The reasons that I like marbles is that they are a. reusable, b. relatively inexpensive and c. your wine remains "your wine." Call me a Purist.
I did not think of pet suppliers. I know you can get them at Craft Stores like "Hobby Lobby" or "Michaels." You may have others in your area, but that is what is around here. Not sure of the pricing at these establishments nor the country of manufacture. That is why I try to buy vintage marbles on eBay.