Hi Neil,
You could alternatively make both rather easily for much
cheaper
Here are the instructions for the zafu:
zafuSewingInstructions.jpg
and for filler you could pickup
kapok for 30 bucks for 5lbs which is plenty of stuffing and, depending on height you require, you can use less than that or
buckwheat hulls for 20 bucks for 5lbs from the
Zafustore.com. Read about these materials even if you decide not to build it yourself so you know what the pros/cons are for either material when buying a cushion or if you build one.
As for a zabuton I made one when I first made a zafu and i just took the size of the area I filled when sitting, added 6" and then cut out the material, stitched it together and filled it with regular pillow stuffing fibers (I gutted the old pillows I was using as zafus). filled it, evenly, stitched it shut. to keep the fibers from bunching up and the zabuton from becoming lumpy you should/could work a spot every say 12", moving the stuffing out of a small area, pushing the material together and sewing through top to bottom to create cells that prevent that fiber from moving.
Or there you could go with a blanket folded to the size of the area you need (when my arthritis acts up I do that, like currently)
Hope this helps some.
Gassho
Shohei