CDNN is having a sale on Ruger Factory 30-rnd Mini-14 Mags. right now at $33.86 each. That's not too bad for factory mag's.
Here's a link to click on:
I lucked out in that my 183-series Mini-14 happily uses any brand of mag. and any brand of ammo that I decide to use. You might look at your Mini's mag. latch and see if it needs to be adjusted. Most mag. problems are because the hole in the back at the top isn't in the right location or isn't the right size. After the mag. latch gets broken in some, it fits the holes in the mag's. better.