If you're after small pieces you could always try spindle blanks. I've bought quite a bit of things from Stiles and Bates, always good service.
For bigger pieces, I've heard surrey timber is good and does courier delivery if you dont live near there. Ill be shopping there soon, as they have some really nice stuff.
All timbers have medullary rays, it's just some like Oak are a lot more pronounced. The classic sapele veneered doors of the 70's usually had the quartersawn grain showing the fine medullary rays.
The best timber I've seen for it was some Bog Oak which had been quartersawn, the rays themselves were silvery in colour compared to the pitch black colour of the rest of the oak. Absolutely outstanding.