Masters Leather has a shoulder holster (#9640) for the PX4. I have never used the shoulder holster, but I have several of the OWB pancake holsters.
MASTER'S Leather SHOULDER Holster Set
...I ride a motorcycle and am looking for a way to carry safely while riding. Any good ideas? I was thinking a nice shoulder rig would suit me fine, but cant find anything decent for under $150.