If Sparks makes a holster for a Shield, the VM2 is by far the best designed and most comfortable IWB holster on the market, if you can wear a cover garment. Unfortunately, the Kydex clips available for tucking are hard to use.

If you must tuck, a single-clip holster works best, since it's hard to keep the shirttail smooth between two clips, or to pull it out of the way when needed. Most tuckable holsters (VM2 excepted) use a single screw on each clip, which works loose in short order. Rather than spend a bundle for tucking, I use Blade-Tech UCH holsters (which use two screws for the belt clip).

The Shield is pretty new, so the best bet is a holster from S&W, or a custom job from Sparks or Ryan Grizzle.

For a tiny pistol like the Shield, you want a very high ride holster, so you can get your fingers around the grip in a hurry. I have a Grizzle "City Slicker" for a Kahr PM45, with the clips in the highest of three positions.