Unfortunately nobody is willing to manufacture "extended safety lever" for Shield. Even Apex, who makes a lot of good stuff for S&W doesn't want to make it.
I saw few people on Internet making it at home, using Epoxy and Dremel.
First you have to "build" small ledge, putting few layers of Epoxy, then shape it to your needs with Dremel. Then color it with sharpie or nail color.
Or... buy M&P9 EZ with thumb safety