My experiences with bamboo garden stakes is that the same 5/16 diameter bamboo can spine out at 25#, 45#, 75# or 95#. If you have a lot of bamboo stakes, I started with 300, and a good spine tester you can eventually find a handful of bamboo stakes that will meet your needs.
Much simpler is to go to Lowes or any other big box home improvement store and pick through their collection of 5/16" poplar dowels. Most of those will spine somewhere between 35# and 45#. Just make sure to pick the ones with the grain running the whole length of the dowel. I can usually find 6 or 8 usable ones after picking through 40 or 50. I have some poplar dowel practice arrows that I have been shooting for 3 years. They have been shot hundreds of times and I have only had to do minor repairs.