You're from SoCal, right? MidwayUSA.com has the 12 round 40S&W on sale for $30, didn't see the 10 rounders, too bad. I personally recommend sticking to the factory (Mec-Gar) mags.
If you're looking for used, cheap mags, browse through the trading post in the WaltherForums. Also send out a WTB post. There's usually someone wanting to get rid of some mags for some reason. I just bought 4 used mags here ($20 for 10 rounders, $30 for 15 rounders).