You don't need one. Just find a spot where you can suspend a weight from the shaft. Mark the spot to the bottom of the shaft, then spin the shaft. You will see that the bamboo will bend lower or come up slighty? If you weren't on the stiff side already. Just make up a few and shoot them without fletching. The ones that shoot most consistent, after you've found the stiff side, you'll fletch those. But a spine tester will be more precise
