Lots of companies use Keller shells. You can also order them through a few different distributors. I recently made a snare drum with a Keller shell that I bought (along with the rest of the parts) from amdrumparts.com. It looks and sounds terrific.
Some people don't like Keller shells, I think mostly because lots of "custom" drum companies use them, so they are nothing special. But I think they are good shells. If 6 or 10-ply maple is what you like, why not use Keller shells? I think they make birch shells too, but they are harder to find to buy to make your own drums.
Most custom drum companies use Keller shells for their maple and birch drums. Birch shells are easy to get. Anderson International sells both. Keller also sells acrylic shells now too in limited colors.
I can't for the life of me understand why people don't like Keller shells, maybe it's because they're the only shell company that sells to the public so everyone uses them. They're good shells. Every once in a while you get a bad shell but that's the same with anything.
The cool thing about he snare drum I ordered was that since I bought all of the parts from American Drum Parts, they cut and drilled the shells for free. Do you know of any place that does this with birch shells? It looks like Anderson International just sells the uncut tubes.