I use 36 grit on my 6X48 Griz for most of my bow work. Sure can thin bamboo in a hurry with this grit, finger tips as well, good thing they grow back on their own.
I get aluminum oxide from an fleay seller named Barbkat. She used to have a company called Discount Abrasives but has gone strictly on line now. She has every size belt as well as disc and flat paper. I pay around $25 for 10 belts from this source.