Arch is also mostly helos. One of their pilots is a DPE in the St. Louis area. I know a few people that have flown for them and for the most part it's on-call and kind of a part time basis as the fixed wings don't fly very often.
If you're looking in the St. Louis area (specifically SUS) and are looking 135 you might want to try Executive Beechcraft. They are almost always hiring and I haven't heard anything bad about working there. Of course it's not air ambulance but they do fly King Airs.