I had my trigger converted by Darrell at Spears in Abbyville, Kansas (800)864-2907. There are probably more Spears 391 triggers around than any other. He does good work, great turnaround time, reasonable price, and he stand behind his work.
You will need to decide how your trigger should be set up and Darrell will do that. Your shooting style and the type of shooting you're going to do will help determine that.
For example, I use my 391 mostly for sporting clays so I need a trigger that doesn't have too light a set or too heavy a release. Also I need a little travel on the sear so it doesn't accidentally go off when I make a rapid gun movement.
If you're just going to shoot singles and handicap, you might want something different.