CDNN has them for $35 Look here: http://www.cdnninvestments.com/rusr99mm17ma.html
AFAIK, no one else makes SR magazines. The only aftermarket mag I trust would be Mec-Gars and I think Mec-Gar makes Ruger's factory mags and won't go into direct competition with their largest customer. Ruger sells the sleeves that fill the space below the grip when a 17 round mag is used in an SR9c