What Shikari said.
You can buy bamboo skewers at most supermarkets, and in different diameters, from "toothpicks" to skewers that are nearly 1/4" in diameter.
I use these as "plugs" so that I can taper grind my shafts (bamboo garden stakes) for glue on points, and for some, plastic nocks.
You can also use hardwood dowels from Home Despot, Lowes, etc. These dowels are sold in numerous diameters so you can find a match for your shafts.
If you drill out the centre of the bamboo, go carefully, or you can split the shaft very easily.