While I do not have a Shield 45, I do have the Shield 9mm and my answer would be a Wright Leather Works custom holster like my "Predator". I have carried my Shield in this one for about 3 years and the fit and retention are still spot on.
Concealed Carry Leather Holsters - Wright Leather Works LLC

[IMG]S&W Shield 003cf3 by Jerry Jaynes, on Flickr[/IMG]