Check anywhere that sells paper, Office Max, Office Depot, Print shop, etc..
Take a micrometer is you have one, most use 0.002" thickness +/- a few tenths.
Look for thin paper like parchment, tracing, etc.. This is usually identified as 9 pound bond and will most likely only be sold in a ream (500 sheets). Don't worry about 100% cotton, harder to find as 25% cotton will work just fine.
Not sure if you have a bullet already or are just searching as bullet diameter and 2 wraps of paper (thickness X4) = bore diameter.