Presently, about the best you can do relative to a "direct replacement" rear sight is to look here:
http://www.tech-sights.com/mini-products/
Bear in mind though, that regardless of who's "drop in" sight you use, both you and the manufacturer are still being forced to "work around" Ruger's crappy sight mounting system(s), which BTW, have only gotten progressively worse during the various stages of Mini-development. Plus, aperture design has much to do with front sight clarity, as well as being able to "quickly spot the target"....and in those regards, the Tech-Sight is really no better than the sight which Ruger provided....plus, there's the chance that the supplied "eye hole" size itself won't be right for your own vision requirements and/or how you want the overall sight picture to look...and if that turns out to be the case (as it was with me), you'll find that buying a click-adjustable sight was only the beginning, not the end.
I guess what it boils down to is whether or not having "click adjustments" is worth the $70 to you because that's the only "sure thing" that you're getting. The rest is perty-much a crap shoot relative to how any certain sight performs "for you".
DGW