Factory mags are the best bet for the Mini-14, and CDNN (mentioned above) always carries them for the cheapest price. But, I have had excellent experience with Thermold 30-round polymer mags also, and new Thermolds can once again be found for sale after a lengthy MIA. A new polymer magazine made by Tapco is out as well. They perform well, but they will allow the top round to pop out when stored loaded to capacity. A cheap dust cover takes care of that problem though...
Ruger factory 20-round mags
http://www.cdnninvestments.com/rumi14220blm.html
Thermold 30-round mags
https://www.budsgunshop.com/catalog/...ducts_id/57875
Tapco 30-round mags
http://www.cheaperthandirt.com/MAG340-1.html