Glock does not make the 30 round mags for the 40 calibers guns, you will have to purchase aftermarket mags, some are Korean made junk. Most aftermarket mags for Glock firearms don't have the most stellar reputation for reliability.
If you are buying it to play with at the range then just understand that you may very well be throwing $$ away. If you are buying it for SD, then stick with Glock brand mags.