When the down ones at Pottery Barn go on sale, I buy some there. They're very nice.

I buy down inserts on Amazon. All sizes. Set of 2 22" here 39.99 https://www.amazon.com/ComfyDown-Feather-Square-Decorative-Stuffer/dp/B0115W9BW4/ref=sr_1_3?ie=UTF8&qid=1509404126&sr=8-3&keywords=down+pillow+inserts+22x22&dpID=41td1ysuEdL&preST=SY300_QL70&dpSrc=srch

Check out stores like Home Goods, TJMaxx, etc. I just switch out their cover for my own.

Also, we generally use a pillow 2" larger than the cover.

IKEA has affordable down insert (also poly filled). They don't have the range of sizes that Pottery Barn has, but still a good selection.

I use the down forms from Crate and Barrel.

foamorder.com has most anything you want including down wrapped foam which IMO is the best choice.

Down wrapped foam is good for cushions, but who uses that for a pillow?

I like Royal Pillow (look under decorator supplies).

Also Wawak (they're having a sale on pillows right now).