Stan Hollenbaugh makes a fine single set, double action trigger that I have used on acouple of guns. Works very well.
http://www.stanhollenbaughgunsmith.com/ 717 259 9271
The S.H. single set trigger can be used either as a single
stage trigger or set.
It is adjustable for both setting pressure & release
pressure when set & no trigger creep. The lock must
have a sear release pressure of five (5) pounds or less.
Warranter against defective parts or workmanship as
long as not altered.
Comes with installation instructions. I am told that it's the
best one available!
$65.00 plus shipping & ins.