Mollenhauer makes a modern alto with an e foot. That’s at least a bit closer to c than the bottom f of most altos. I have one in palisander/rosewood and it’s a marvelous instrument.
If you find the tenor too large in the finger spread, it is possible to have keys installed for the 3rd and 4th fingers (I assume that you have the c and c# keys already?) that makes the spread much more manageable. Also, it is possible to have the neck “knicked”, or bent, which brings the hands up higher and thus makes the spread more comfortable.
Different tenors have slightly different finger spacings as well. Check out this chart on Bill Lazar’s website. http://www.lazarsearlymusic.com/finger_hole_distances.htm
As far as a market for an alto with a low c, I suspect that the tenor already fills that niche to overflowing.
So once you learn the alto, transposing should not be a problem.