Sharpening supply's has a Hard Black Ark 11.5" X 2 1/2" X 1/2" for $129.99. I suspect they are from Dan's and are replacement stones for Norton IM313's. They also have a stone holder that will fit them.
Just my $0.02, but I've been honing for about a year now and am at the point that I regularly get good edges (in my own opinion). .....
I just resurfaced 2 old family heirloom washita stones. One my Grandfather taught me how to hone his razor on when I was 12 years old. They are both 2" stones, and served to hone around 150 years of razors and wood plane blades.
2 & 2 1/2" stones seem to be prevalent & do a perfectly fine job, given knowledge & a little expertise from bygone ages.