I went on Gunbroker.com and put in a search for magazines... GunBroker.com - Online Gun Auction / Firearm Auction is the home page. Then I went to gun parts, magazines, typed in bersa and searched for a deal. I have used ProMags, they seem to work okay, but they are not factory original. They are all kind of costly, about 25 per magazine. Auction Arms is another place to look. There is also the Bersa webpage and Numrich gun parts.
Good luck.