Bill - I'm not aware of any flat slick 26 inch tires.
But... you do
NOT want a roller that has a concave surface to 'match' the radius of the tire. At all.
Here's the problem. If you have a roller that is 1.5 inch at the widest part, and 1.25 inch at the narrowest part, and it's placed in contact with the tire:
the 1.5 inch portion is trying to make the tire rotate faster than the 1.25 inch portion of the roller at the same engine RPM ... about 20 percent faster!!!
So, either the 1.25 inch part of the roller will be grinding away at the crown of the tire, or the 1.5 inch portion will be grinding away towards the sidewalls of the tire, or BOTH will be grinding away at the tire.
In any case, your tire will be ground away soon, AND you'll be wasting a huge portion of your available power. Not a good situation.