JB,

You can get what you need from American Home & Habitat Inc.-
http://www.ahh.biz/index.htm - they have the mesh, grommets and
nylon webbing. It's been a while since I've ordered from them, but I
used the 0.75mm vinyl coated mesh. I still have enough left to make
another seat if you're interested. It's grey or "silver" as I like
to call it. What's nice is that you can get the nylon webbing in a
color to match. If you know what color your bike is going to be,
it's a good way to customize the look. You've nine colors to choose
from with the 0.75mm:

http://www.ahh.biz/fabric_catagories/vinyl_composites.htm
http://www.ahh.biz/products/Vinyl%20coated%20polyester%20mesh%
20.75mm%20FVM075.htm

Not having a heavy duty sewing machine, I took the webbing and mesh
to a local leather place and had them sew it up for me. It cost me
about $20. You could get any tailor to do the same thing. Usually a
good dry cleaners will have someone that does alterations. They can
handle it. If there is an old school shoe repair shop near you, they
know how to work with heavy material and webbing too, but I'd try a
leather coat/leather goods shop that does alterations first.

Jim