Feathers for Helmets

[Agricola]

I've got an Imperial-Gallic F helmet (Besançon ) and I've been wondering what type of feathers to use for the two crest holders on the side of the helmet. I originally got crow's feathers, but I've ditched them as I can't find any information to verify their historicity. Can anyone give me any information on what types of feathers that were used, whether they were dyed, and where I could purchase them - if they aren't readily available around the British countryside.

Thank you in advance for your help.