If you are sure it's necrotic ringspot, the refer to the NTEP tests for necrotic ringspot. It is found on page 110 of this pdf: http://www.ntep.org/data/kb05/kb05_11-10f/kb05_11-10f.pdf
Bewitched is near the top, is a very shade tolerant variety of KBG, and has excellent color and turf quality.

Tall fescue is generally not affected by NRS and will also do very well in shady areas. How much direct sun does the area receive on a typical summer day?

Thanks for the link. [110 pp of .pdf!!!]

There are two areas. One gets little direct sun, shaded by large ash trees, but enough to grow grass successfully when the disease isn't active. The other is shaded mostly by a large crabapple and gets some direct sun early and late.

It will help tiemco make recommendations if you give the number of hours on an average sunny summer day that the areas get direct sulight.

The first area probably doesn't get more than an hour of direct sun.

The other is so various, with different shade sources, I couldn't really say, except that on average it gets more direct sun - 3-4 hours in some places, less in others.

My real question, though, isn't exactly what specific variety of grass, but where I can purchase such grass seeds in homeowner quantities.

tiemco posted this a few weeks ago:

"Posted by tiemco 6ct (My Page) on Fri, Aug 3, 12 at 17:12

There are a bunch, but here are a few of the good ones.
Pawnee Buttes
The Hogan Company
Williams Seed (ask for Dennis)
Oliger Seed
Chas Hart Seed
F.M. Brown's Sons "

Thanks a lot. I searched for necrotic ring spot but didn't find that post.

That area that receives about an hour of direct sun will probably not support TTTF and forget about KBG. The only grass that might work is poa supina, but fine fescues may have a chance too.

With 3-4 hours of direct sun you can grow TTTF, but select shade tolerant ones (consult the NTEP for TTTF). Bewitched might work, but it will probably not be at its full potential and could struggle in the shadier spots.

I know I can grow fine fescues, but they seem to be susceptible to ringspot. The diseased areas are predominantly fine fescue.