The Vicker's slide stop that ClydeFrog mentioned is excellent. I have it on both my Glock 20 and Glock 21 - and will soon add it to my Glock 19 and Glock 26, replacing the OEM extended slide stop. It works great and does feel very similar to the M&P slide stop which is a good thing for me, running both platforms. Price is great and shipping from Amazon is quick.
Here are a few pics.