Midwayusa has factory Ruger magazines on sale right now. $20 for 20 rounders, and $26 for 30 rounders.
Ruger makes them.
There has been many aftermarket mag suppliers for the MINI-14, but after trying several of them in the mid 1990's, I gave up. I could never figure out why they wouldn't work consistently. Later I bought my AC556, I did try one of the aftermarket mags, it didn't work. After that, I just bought nothing but RUGER mags ! And Today, I'm still tired of wasting money buying the AM mags. I only have Ruger mags now !!!