MidwayUSA has some of the flat base plate 8 round mags for $33...the curved base plate are out of stock (but you can backorder.) They also have the curved base plate for about $3 if you wanted to add it to the flat base plate mags. They have some other brand mags for $22 or so, but I've never heard of them and the reviews aren't stellar.

I bought my extra mags from Midway (they had the curved baseplate mags in stock then) and I am very pleased with them.