Where do I buy itomaki?


Hello everyone, for some reason or another I ordered a shamisen online at a really cheap prize, no itomaki thou. I recieved it today and after assembling it I noticed that the sagane at the tenjin are too big for the itomaki I have at home or… maybe the itomaki I have are too thin for the sagane of my new shamisen.

Either way, my question is where on Earth can I buy thicker itomaki? I rather buy plastic ones because they are cheaper, thanks in advance.