Well, first of all...anything that's available in any other state should be available here.
Probably the easiest DIY solution to find besides paint is blackout cloth (drapery linining). Any fabric store will have it for around $6/yard. Build a frame of suitable size, stretch and staple. Google will turn up a number of hits for "stretching canvas frames". Searching HTF will probably turn up some play-by-play instructions, complete with photos. BOC is sometimes used to indicated blackout cloth.
Paint can be as simple as a flat white or as complicated as 4+ part mixtures with exacting ratios. My first DIY screen attempt was a sheet of Gatorfoam painted with Behr Ultrawhite Flat. Some people paint the screen directly onto a room wall, but it needs to be very smooth and free of imperfections. Otherwise, you can use any smooth suface the paint will adhere to. Something like Masonite hardboard is popular, but at 1/8" thick, will probably need a stiffener of some sort.
I'm currently using a BOC screen myself, but plan to upgrade to a Wilsonart Designer White laminate screen. DW can be ordered from Home Depot/Lowes and usually local countertop makers. Testing on other forums have shown it to be an almost perfectly neutral white and claims as high as 1.24 gain. The screen shots look pretty good as well. You'll need a substrate of some type. I plan to get a piece of 1/2" plywood, cut it down to size, and laminate the DW to it. To save some weight, I may cut some big circles out ofthe plywood, but it probably won't be necessary. Figure $100ish for this solution.
For me, 100" at 12' would be too large. Currently I'm about 84" at 12' and plan to bump to 89" when I go DW and upgrade the projector.
This calculator will give you the SMPTE and THX recommendations for size/distance. According to SMPTE my 89"@12' is the max recommended distance, but that's ok with me...THX says I'm too far away or my screen is too small.
You could alwasy use some blue painters tape to mock up the screen size on your wall to get a feel for what you'd prefer.
-Brent