Hi,
For soft hackles I've used, all with success, starling (as previously mentioned; very nice), cockpheasant wing feathers (there are some nice chesnut brown feathers on the "shoulders" and "elbows" which are great), hen pheasant wing feathers, woodcock wing feathers (these are a nice earth tone as well), and hen feathers. I've also used Hungarian Partridge, which are great.
I would think rough grouse would be an excellent source if you, or anyone you know, hunts them.
- Jeff