You need the original factory SKS magazine. I dunno what to tell you if you bought it like that. It sounds like the guy you bought it from just threw it away after he converted it.

I did a quick google search and found a guy that sells them. You have to scroll down to the bottom of the page and look for Original 10rd Magazine 20.00. Here's the site and I hope it helps http://www.surplusfirearms.com/sksrifle.htm. I'm sure you can find em on other sites if you look around a bit. This is what it looks like