Goldenwitch.com sells the best stuff by airmail. It's flyrod grade but there is a fair amount that goes through their hands that is too small for rods, and would probably be perfect for bamboo bikes, as the sub 1.5" stuff isn't good for rods.

At one time Charles H Demorest company of new jersey was the sole importer of the high end tonkin cane, and this may well still be the case. You might be able to contact them with your inquiry, they are a wide spectrum importer of Asian goods for generations, so I don't know if they have a website, and they used to have minimums, but they could probably put you in touch with any local distributors.

By the way, a lot of dry bamboo has a grenish gold tinge to it. To reach peak performance it needs to be "heat treated" which consists of baking or flaming the culm to drive off excess moisture. There is a lot of info on all this stuff in various bamboo rod texts. Nicely backed bamboo is stronger and has a chocolate colour.

When I made my bamboo fenders, I needed to steam the bamboo (which also drives out excess moisture contrary to what one might imagine). The resulting colour while not dark was very pleasing, like late season golden grass.