http://www.astroflight.com would be a good place to start looking. You might consider writing Astro Bob an email too.
I have the FAI 05 geared and used to run it on 8 cells with a 12x8 folder in a 2M sailplane, it climbed very well. [8D] I've retired it and since gone brushless geared and 7 cells for WOW climb.
For direct drive though I think you will be limited to an 8x4 prop on 8 cells, maybe a .15 size plane?