Unfortunately I am not familiar with anyone who sells by the inch either. Sorry I can't chime in with more productive information, but I would say that NJBillK is right that you might try NJ Steel Baron.
I have had to purchase stock of several steels, and would love to know about someone who would be willing to sell if by the inch, so if you do find someone who will be able to do it, please let us know.
Purchasing full bars is way too much for someone like me who only does it for testing and can't make knifes for sale at all.