I saw some new at a gun show they wanted $40 Shop Ruger has them for about $35 plus shipping. I found one at the Army PX in Columbia SC. for $25. You might want to search the classifieds parts and accessories forum.
If you registered your gun on-line with Ruger, they give you a 20% discount coupon for accessories and clothing at Shop Ruger.com. $35 less 20% makes it $28 plus shipping. I'm going to order a few mags soon.