Down-lite sells 800, 700, and 600 FP down by the pound:
Feathertex is the cheapest I have seen online, but they have no stock until this fall.
Feather Industries, in Toronto, sells in quantities as small as two pounds at a premium to their normal minimum of twenty pounds. They were very nice in pointing me to down-lite (above), which was more economical for an order of one pound. Feather Industries also suggested checking with Pacific Coast Feather, and United Feather & Down. I did not call those last two.