Good luck. I used to have both a Vz52 and a Vz52/57 and couldn't find 52/57 mags, no matter how hard I looked. I've actually only seen one, and that was with a rifle at the local shop. They still have the rifle, but they want entirely too much for it, ~$450, IIRC.
The Vz52, 7.62x45, mags will work just fine with the Vz52/57. They're just not the exact right mag. In fact, many Vz52/57's were actually converted 52's, complete with Vz52 mag. (They just stuck a sleeve in the chamber, so that the shorter round would fit properly.)
If you check Gunbroker, Auction Arms, or Guns America, you will find some sellers with Vz52 mags. There are generally two dealers with them. One is in TX, the other in CA. I had a bad experience with the seller in CA, and wouldn't recommend him. Dealing with the other seller was great, however.