You could start by playing with the half-dozen or so blur filters under the video effects already in Premiere. I also use the Rack Focus from NewBlue FX. Free trial here: http://www.newbluefx.com/video-essentials-ii.html
To get soft focus you do not need a plugin.
Copy clip and superimpose on track 2, add Fast Blur and set it to about 30. Lower the opacity between 50 and 70. Done.
If you want certain area's to be soft focus use a matte.