You would be able to find 8 graduated solid colors in hand dyed fabrics. Having looked for graduations of colors, I doubt you will be able to find 12 that will work for you. Some will look the same from a distance. You might be able to do it with tone on tones but again 12 is an awesome number.
In my search I got color cards from several manufacturers and had difficulty putting together 6. So a tru graduation would be the hand dyed fabrics - but they usually come in 8 shades. A good source to look would be Cherrywood Fabrics
I don't know if this is the name but Cherrywood is the name.
Have you considered dying the fabric yourself.