Every blue moon you can get them for $15 each. Keepingshooting.com and Copes had them about a month ago but they've sold out. Right now they're about $25 each. Here are a few resources:
http://www.ohioordnanceworks.com/vz-2000-magazine-new
(it's listed as VZ2000 magazine, but that's just the name of their variant rifle. I'm 99.999999999% confident it's the same as the VZ 58 magazine, but all them to make sure).
http://www.robertrtg.com/store/pc/VZ58-c302.htm?pageStyle=h&ProdSort=3&page=2&idCategory=302
http://www.clearviewinvest.com/vz58leathermagpouchand430rd762x39magazines.aspx
I think that the last one is probably the best deal as it comes with a pouch. Gunbroker has listings from time to time, sometimes for 8, 10, or more lots.
Don't be afraid of used VZ 58 magazines. I have some that look beat to hell but work perfectly.