You do not need to have it placed on letterhead if the feathers come from an aviary, zoo, pet store, or breeder it is best to have what feathers (including what type and how many of each type) you got from them listed on their letterhead. However, fir feathers from friends, relatives, etc. who do not have a business dealing in the birds, all you need is a list of the feathers (including how many of what type from what bird) that you received from the person and the date you received them along with the person's signature.
Notarizing is nice but it is not necessary for documenting the feathers are from a domesticated source. Likewise, a picture of the feathers is also nice but not needed.
As you can see, documentation is really very easy to get.