Assuming you mean unstraightened and not heat treated, check out your local garden supply store. The big box home improvement stores are another possibility. You just need to look around to see who carries a good supply of garden stakes in the right diameter so you can fish through for the good ones.
If you want bamboo that has been straightened, heat treated, the nodes knocked down, and the skin taken off - essentially bamboo arrow shafts - then there are a couple guys on the online auction site that sell them as well as a few archery vendors. The auction guys are far less expensive and they're the same people the archery stores buy them from.
Guy