For the shinken that I use for iaido I use susudake, cut and filed to fit the tsuka and the mekugi-ana.
Susudake is smoked bamboo, thst is strong and flexible, far stronger that chopsticks bamboo. I would not trust chopsticks bamboo for my iaido practice. On the other hand if you do not intend to actually use it normal bamboo will do.
I usually purchase mine here https://www.namikawa-ltd.com/product/131.
Regards
Luca