Well, the general rule on feathers is - shrink the size of the bird and you will find smaller feathers.
I do, however, offer with this post one alternative approach. One more tool in our toolkit, as it were. Flytying is all about having choices.
First a sample fly, just a simple generic softy, but one shrunk down in size. The challenge often is to find a suitable size hackle.

Proton
Hook: Mustad 540L #28 (or equivalent)
Thread: Benecchi 12/0, grey
Hackle: Single barb taken from the webby part of a Whiting Brahma hen neck
Body: SLF Minky dubbing, #22 mayfly yellow
Here is a picture of the single barb which forms the collar on the Proton, next to the finished fly

The back of the Whiting Brahma hen neck

Detail image of where to locate the barbs

The barbs highlighted

Cheers,
Hans W